A Hybrid NOMA/OFDM Model for Next Generation Communication Systems

A Hybrid NOMA/OFDM Model for Next Generation Communication Systems | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article A Hybrid NOMA/OFDM Model for Next Generation Communication Systems Abdulmutalib A-Wahab Hussein, Ammar Bouallegue This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3879685/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 8 You are reading this latest preprint version Abstract Visible light communication (VLC) is an innovative optical wireless communication (OWC) technology that can provide both lighting and high-speed wireless data transmission. Another advantages of VLC system that allow for a wide range of applications are its fast data rate, reliable communication channels, and interference protection against electromagnetic (EM) waves. Light Emitting Diodes (LEDs) are used as transmitters while Photodetectors are utilized at the receiver side. The LED has to be supplied with a positive real valued signal. Orthogonal frequency division multiplexing (OFDM) uses multiple subcarriers with orthogonal frequencies to enhance the spectral efficiency of the system. There are two types of unipolar OFDM used to obtain real-valued and positive signal, they are DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM). A hybrid design NOMA/OFDM is introduced in this paper to be able to study the performance of the VLC system. Three users are positioned in different spots throughout an interior environment. User1 is considered as the farthest user with lower channel gain while user3 is the near user that has high channel gain. The bit error rate (BER) is estimated for the three users vs. the SNR with different power allocation coefficients. The results show that NOMA/ACO-OFDM performs better than NOMA/DCO-OFDM. Also, the BER of the three users converges to each other with the increasing of the SNR for the two models. Visible light communication (VLC) Orthogonal frequency division multiplexing (OFDM) Nonorthogonal multiple access (NOMA) Bit error rate (BER) Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 1. Introduction An optical technique named light fidelity, or Li-Fi, is primarily based on photodiodes (PD) [ 1 ]. Li-Fi offers faster transmission speeds, higher bandwidth, and the ability to operate in environments that are vulnerable to electromagnetic interference [ 1 ]. This is because photodiodes (PDs), or Light Emitting Diodes (LEDs), consume very little energy. In contrast to Wi-Fi, Li-Fi does not create electromagnetic interference [ 2 ]. Visible light communication (VLC) recently has been proposed as a viable solution for indoor wireless communication because of its wider bandwidth, which does not violate the RF spectrum, its efficient energy, and its ability to provide ubiquitous communication [ 3 ]. VLC presents significant promise for improving wireless internet access in the future. VLC is an acronym for optical wireless communication (OWC) utilizing the visible light spectrum ranging from 380–780 nanometers as shown in Fig. 1 [ 4 ]. Each user in an orthogonal multiple access (OMA) system, such as orthogonal frequency division multiplexing (OFDM), is assigned an orthogonal frequency [ 3 ]. By using OFDM, high-speed data transfer over a dispersive channel is made possible and the inter-symbol interference (ISI) can be eliminated by adding a small redundancy, namely the cyclic prefix (CP) [ 5 ]. Nevertheless, commercial OWC do not employ OFDM. This is because only unipolar signals may be conveyed in VLC systems that employ intensity modulation (IM), but OFDM signals are bipolar [ 6 ]. In this paper, two types of unipolar OFDM have been employed. They are asymmetrically clipped optical OFDM (ACO-OFDM) from and DC-biased optical OFDM (DCO-OFDM) from [ 7 ]. The bipolar signal of ACO-OFDM is clipped to zero while in DCO-OFDM, a DC bias is added to remove the negative values. Non-orthogonal multiple access (NOMA) was suggested as a potential radio access method for 5G cellular networks [ 8 ]. NOMA enhances the spectral efficiency of the system by enabling multiple users to utilize all the available frequency and temporal resources simultaneously [ 8 ]. In essence, NOMA differs from other multiple access methods that offer subscribers orthogonal access in terms of time, frequency, code, or space. In NOMA, all users work simultaneously in the same band, differentiated only by their power levels [ 9 ]. As illustrated in Fig. 2 , K users can share the entire BW of OMA system whereas in NOMA, the entire transmitted power is distributed among the K users. The transmitter side of the NOMA system uses superposition coding, and the receiver employes successive interference cancellation (SIC) for distinguishing users in the uplink and downlink [ 10 ]. Quadrature amplitude modulation (QAM), one of the higher-order modulation techniques, can be applied to significantly improve the spectral efficiency of NOMA based VLC systems. Since indoor VLC system uses intensity modulation/direct detection (IM/DD), QAM signal cannot directly be applied to VLC since it produces bipolar complex-valued symbols. To overcome this limitation, we could make use of O-OFDM technology, which generates real-valued signals in the time domain by utilizing the concept of Hermitian symmetry [ 11 ]. A NOMA system for indoor VLC environment under realistic channel conditions is implemented in [ 12 ] and the results revel the superior performance over orthogonal frequency division multiple access (OFDMA). In [ 13 ], an ACO-OFDM NOMA model was presented in order to compute the BER analytically and through simulation vs. the SNR. A user with a higher power allocation has a lower bit error rate (BER) than one with a lower power allocation. In this work, hybrid NOMA ACO-OFDM and NOMA DCO-OFDM are proposed to serve three users located in a VLC cell at different distances away from a single transmitter (LED) situated on the center of the ceiling. Each user is assigned different power allocation conditions. The BER of the three users is estimated as a function of the SNR. 2. System Model In this paper, a VLC cell that serves 3 users by a single LED is assumed. The LED is located on the ceiling center of a room. The users are located varying distances apart from the transmitter. A downlink channel model for line of sight (LOS) is taken into consideration. Shadowing and reflections are not taken into account. 2.1 Channel Model for VLC Indoor Environment The channel model shown in Fig. 3 is considered. The point-to-point VLC system is expanded in this model to support K users. L is the vertical distance, in meters, between the LED and the receiving plane, while \({r}_{k}\) represents the horizontal separation from the LED. The angles of incidence and irradiance are \({\psi }_{k}\) and \({\varphi }_{k}\) respectively [ 14 ]. Assume that all users have been ordered according to their channel quality [ 15 ]: $${h}_{1}<{h}_{2}<\dots {a}_{2}>\dots >{a}_{k}$$ 2 where \({h}_{k}\) represents the channel gain of LOS link between the LED and the k th user [ 16 ]: $${h}_{k}=\frac{\left(m+1\right)A {R}_{PD}}{2\pi {d}_{k}^{2}}{cos}^{m}\left({\varphi }_{k}\right){ g}_{f}\left({\psi }_{k}\right){ g}_{c}\left({\psi }_{k}\right)cos\left({\psi }_{k}\right)$$ 3 Where A stands for the PD's detection area, R PD for the PD's responsivity, and \({ g}_{f}\left({\psi }_{k}\right)\) for the optical filter's gain at the receiver. The distance between the k th user and the LED is \({d}_{k}=\sqrt{{r}_{k}^{2}+{L}^{2}}\) ; m and \({ g}_{c}\left({\psi }_{k}\right)\) denote the Lambertian emission order of LED and optical concentrator's gain respectively, are given by [ 17 ]: $$m=-\frac{1}{{log}_{2}\left[cos\left({\phi }_{1/2}\right)\right]}$$ 4 $${ g}_{c}\left({\varphi }_{k}\right)=\left\{\begin{array}{cc}\frac{{n}^{2}}{{sin}^{2}\left({\psi }_{FOV}\right)}& 0\le {\varphi }_{k}\le {\psi }_{FOV} \\ 0& {\varphi }_{k}>{\psi }_{FOV}\end{array} \right.$$ 5 In the equations 3 and 4 , \({\phi }_{1/2}\) , \({\psi }_{FOV}\) and n denote the LED's half-power semi-angle, the receiver's angle field of view (FOV) and the refractive index constant respectively. The channel gain can be written as follows, assuming the PDs are continuously pointed toward the ceiling: $${h}_{k}=\frac{\left(m+1\right)A {R}_{PD}}{2\pi }\frac{{L}^{m+1}}{{\left({r}_{k}^{2}+{L}^{2}\right)}^{\frac{m+3}{2}}}{ g}_{f}\left({\psi }_{k}\right){ g}_{c}\left({\psi }_{k}\right)$$ 6 2.2 NOMA-OFDM MODEL IN VLC SYSTEM Figure 4 depicts the entire block diagram of the suggested system model. The data of each user is modulated using M-QAM modulator. A power allocation strategy is applied to the three users. The users have been assigned to different positions within the VLC cell, with user 1 being the farthest and user 3 being the closest, as shown in Fig. 5 . in which \({d}_{1}>{d}_{2}>{d}_{3}\) . The channel gains of the users are \({h}_{1}>{h}_{2}>{h}_{3}\) according to the position of each user. The power allocation factors of the three users are \({a}_{1}\) , \({a}_{2}\) and \({a}_{3}\) so that \({a}_{1}>{a}_{2}>{a}_{3}\) . Lower power should be provided to the user who has higher channel gain, and high power has to be delivered to the user with lower channel condition [ 18 ]. Combining the signals of all users gives [ 19 ]: $$X=\sum _{k=1}^{K}\sqrt{{a}_{k}{P}_{t}}{s}_{k}$$ 7 and for three users: $$X=\sum _{k=1}^{3}\sqrt{{a}_{k}{P}_{t}}{s}_{k}={P}_{t}(\sqrt{{a}_{1}}{s}_{1}+\sqrt{{a}_{2}}{s}_{2}+\sqrt{{a}_{3}}{s}_{3})$$ 8 The signal X obtained in Eq. 8 is complex-valued which cannot be applied to the LED because the LED has to be supplied by real-valued and positive signal. Hermitian symmetry can be utilized before the inverse fast Fourier transform (IFFT) to obtain real-valued set of data symbols [ 18 ]. \({X}_{0}\) and the middle \({X}_{N/2}\) subcarriers are set to be zero [ 19 ]. $${X}_{H\_DCO}=\left[0, {X}_{1},{X}_{2}\dots {X}_{\frac{N}{2}-1},0, {X}_{\frac{N}{2}-1}^{*},\dots {,X}_{2}^{*},{X}_{1}^{*}\right]$$ 9 $${X}_{H\_ACO}=\left[0, {X}_{1},0,{X}_{3}\dots {X}_{\frac{N}{2}-1},0, {X}_{\frac{N}{2}-1}^{*},0,\dots ,{X}_{3}^{*},0, {X}_{1}^{*}\right]$$ 10 Then, the output of the IFFT is the time-domain real-valued signal \({x}_{n}\) [20]: $${x}_{n}=\frac{1}{N}\sum _{k=0}^{N-1}{X}_{k} {e}^{j\frac{2\pi nk}{N}} 0<n<N-1$$ 11 Where N is the FFT/IFFT size, and \({X}_{k}\) is the k th subcarrier symbol. A parallel to serial (P/S) converter converts the IFFT output to serial form, and then a cyclic prefix (CP) of a particular length is then added for avoiding inter symbol interference (ISI) by transforming the linear convolution into a circular convolution. A direct current (DC) is added in the case of DCO-OFDM and zero clipping of negative values when ACO-ODDM is used to gain a unipolar real-valued signal to supply the LED. Figure 6 illustrates the DCO-OFDM and ACO-OFDM modulators. At the receiver side, the received signal at each PD (user) is [ 18 ]: $${y}_{k}\left(t\right)={h}_{k}x\left(t\right)+{n}_{k}$$ 12 where \({n}_{k}\) denotes the additive white Gaussian noise while \({h}_{k}\) represents the channel response between the LED and the k th user. The optical signal detected by the PDs is converted into an electrical form. The PD's output signal is filtered before being digitally transformed. An S/P circuit is used to convert the signal from serial to parallel after the CP has been removed. Following that, the FFT is utilized to obtain a frequency-domain output from the received time-domain signal [ 21 ]. $${X}_{k}=\sum _{n=0}^{N-1}{x}_{n} {e}^{-j\frac{2\pi nk}{N}} 0<k<N-1$$ 13 User 1, with the highest transmission power, recovers its data directly without SIC process because it views the signals of the other two users as noise. User 2 employs SIC to eliminate interference from user 1. In order to detect its data, user 3 can eliminate the interference due to user 1’s and user 2’s signals via employing SIC process twice. Figure 7 explains the decoding process of the three users. The flowchart of the proposed model is shown in Fig. 8 . The BER can be defined as the ratio of the received corrupted bits to the total number of transmitted bits [ 21 ]: $$BER=\frac{recieved corrupted bits}{Total nomber of bits}$$ 14 3. Results and Discussion In this paper, the BER of the three users is estimated as a function of the SNR with equitable constellation order (M1 = M2 = M3 = 4). Figures 9 , 11 and 13 show the BER for NOMA/DCO-OFDM model while Figs. 10 , 12 and 14 for NOMA/ACO-OFDM model. The three users share the total transmitted power of the LED inequitably. Three cases were chosen to distribute power among users. The first case the power allocation coefficients are \({a}_{1}=0.65\) , \({a}_{2}=0.25\) and \({a}_{3}=0.1\) for user1, 2 and 3 respectively. The BER of the three users is very high for the two models. The performance has got better when the power allocation coefficients are \({a}_{1}=0.7\) , \({a}_{2}=0.22\) and \({a}_{3}=0.08\) . The lower BER is obtained when \({a}_{1}=0.75\) , \({a}_{2}=0.2\) and \({a}_{3}=0.05\) . In order to compare the performance of the three users, the BER of the three users is plotted as shown in Figs. 15 , 17 , 19 for NOMA/DCO-OFDM and Figs. 16, 18, 20 for NOMA/ACO-OFDM. Figures 15 and 16 shows high BER for the two models for power allocation coefficients \({a}_{1}=0.65\) , \({a}_{2}=0.25\) and \({a}_{3}=0.1\) . The performance is enhanced as the \({a}_{1}\) increases. The results illustrate that the users’ BER converges to each other when the SNR increases. Figure 18 shows that the BER at SNR=35 dB is between 10 − 5 and 10 − 6 for NOMA/DCO-OFDM comparing with Fig. 20 for NOMA/ACO-OFDM the BER reaches 10 − 6 at SNR=34.5 dB. Also Fig. 19 shows that the BER reaches 10 − 6 at SNR=28.6 dB for NOMA/DCO-OFDM whereas for NOMA/ACO-OFDM at SNR=27.2 dB the BER reaches 10 − 6 . 4. Conclusion In this paper, two models NOMA/DCO-OFDM and NOMA/ACO-OFDM are presented to estimate the BER as a function of the SNR of three users are located in an interior environment of a VLC system. Three strategies for power allocation coefficients are considered. The first strategy where the power allocation coefficients for the three users are \({a}_{1}=0.65\) , \({a}_{2}=0.25\) and \({a}_{3}=0.1\) shows poor system performance for the two models. A better BER is obtained for the second ( \({a}_{1}=0.7\) , \({a}_{2}=0.22\) and \({a}_{3}=0.08\) ) and third ( \({a}_{1}=0.75\) , \({a}_{2}=0.2\) and \({a}_{3}=0.05\) ) strategies. the first strategy. The results illustrate better performance of NOMA/ACO-OFDM than that of NOMA/DCO-OFDM. As the SNR grows, the BER of the three users converges. Declarations Author Contribution Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission. References Huang, T.-C.Y.W.-T., Lee, W.-B.: Visible Light Communication System Technology Review: Devices, Architectures, and Applications, Crystals, vol., 11, No.1098, Sep. 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Sci. 131 , 1074–1080 (2018) Tabarek, H., Abood, Ismaiel, H.: BER Performance for Downlink NOMA, Wasit Journal of Engineering Sciences, vol. 10, no. 2, Mar. (2022) Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Reviews received at journal 18 Mar, 2024 Reviewers agreed at journal 11 Mar, 2024 Reviewers agreed at journal 21 Feb, 2024 Reviewers agreed at journal 21 Jan, 2024 Reviewers invited by journal 21 Jan, 2024 Editor assigned by journal 19 Jan, 2024 Submission checks completed at journal 19 Jan, 2024 First submitted to journal 19 Jan, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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4","display":"","copyAsset":false,"role":"figure","size":81343,"visible":true,"origin":"","legend":"\u003cp\u003eThe Complete Model of NOMA/OFDM\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/b0e73391f5ca82ea57c306ba.png"},{"id":50103838,"identity":"2bceb04e-f41c-441f-bc05-06b6170b4d10","added_by":"auto","created_at":"2024-01-24 15:28:46","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":35328,"visible":true,"origin":"","legend":"\u003cp\u003eThree users in VLC Cell.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/cbb12356fb392bd525673cd8.png"},{"id":50103850,"identity":"bbaa45a3-1899-4cad-a797-413d8740829c","added_by":"auto","created_at":"2024-01-24 15:28:46","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":141875,"visible":true,"origin":"","legend":"\u003cp\u003eDCO-OFDM and ACO-OFDM Modulators\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/d7e960b442b7d4eb5e1e3cde.png"},{"id":50105547,"identity":"3ccdbff6-d26c-4037-a647-76bf0b6620f7","added_by":"auto","created_at":"2024-01-24 15:44:46","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":39621,"visible":true,"origin":"","legend":"\u003cp\u003eThe Process of SIC.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/857d64c6c021801b909510e0.png"},{"id":50104885,"identity":"29e918d9-bea1-436f-bd79-3d05b3ce697d","added_by":"auto","created_at":"2024-01-24 15:36:46","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":26800,"visible":true,"origin":"","legend":"\u003cp\u003eFlowchart of the Proposed System\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/9240fd368eb37b4ed34d1fdf.png"},{"id":50105546,"identity":"a0887fa5-89ea-48ad-b0c9-01290d9b6b5c","added_by":"auto","created_at":"2024-01-24 15:44:46","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":23179,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of User 1 NOMA/DCO-OFDM\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/38301dc204945efedca882d2.png"},{"id":50104889,"identity":"9d363644-83c6-4593-ba8b-8de406fb221a","added_by":"auto","created_at":"2024-01-24 15:36:46","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":23741,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of User 1 NOMA/ACO-OFDM\u003c/p\u003e","description":"","filename":"image10.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/45c0373e9ffe1e61440030e1.png"},{"id":50104891,"identity":"c8812de8-69c6-44c3-890e-0a29f88b8c15","added_by":"auto","created_at":"2024-01-24 15:36:46","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":21022,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of User 2 NOMA/DCO-OFDM\u003c/p\u003e","description":"","filename":"image11.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/0a370c058df80c69649f2bc4.png"},{"id":50104888,"identity":"187a529a-8393-494f-b3e0-1db7192faa7c","added_by":"auto","created_at":"2024-01-24 15:36:46","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":22972,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of User 2 NOMA/ACO-OFDM\u003c/p\u003e","description":"","filename":"image12.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/ee6bc7cc724612e3a29bdef4.png"},{"id":50103847,"identity":"06a17c6d-8921-4e40-83c8-28ff5a1bc50e","added_by":"auto","created_at":"2024-01-24 15:28:46","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":22612,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of User 3 NOMA/DCO-OFDM\u003c/p\u003e","description":"","filename":"image13.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/6b7edd4ea3026c456e2d09f0.png"},{"id":50105548,"identity":"4b5f8298-3ce9-43f6-a9f7-e8632e879e0b","added_by":"auto","created_at":"2024-01-24 15:44:46","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":23014,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of User 3 NOMA/ACO-OFDM\u003c/p\u003e","description":"","filename":"image14.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/db21ae198e32b9238c34f10a.png"},{"id":50104893,"identity":"19d055bc-cb94-42fb-b5ed-9b82a530d032","added_by":"auto","created_at":"2024-01-24 15:36:46","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":18957,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of Users 1, 2 and 3\u003c/p\u003e\n\u003cp\u003eNOMA/DCO-OFDM (a1=0.65, a2=0.25 and a3=0.1)\u003c/p\u003e","description":"","filename":"image15.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/21a582542bc9aaaf030dcba4.png"},{"id":50103845,"identity":"f1a42f76-5e87-4a9f-a2b2-7ae820fefef8","added_by":"auto","created_at":"2024-01-24 15:28:46","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":18934,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of Users 1, 2 and 3\u003c/p\u003e\n\u003cp\u003eNOMA/ACO-OFDM (a1=0.65, a2=0.25 and a3=0.1)\u003c/p\u003e","description":"","filename":"image16.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/4c6fa0751158ab3fbdd0c5fc.png"},{"id":50105549,"identity":"5838fb15-4a43-43a9-8726-fe2a6e812fcd","added_by":"auto","created_at":"2024-01-24 15:44:46","extension":"png","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":21371,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of Users 1, 2 and 3\u003c/p\u003e\n\u003cp\u003eNOMA/DCO-OFDM (a1=0.7, a2=0.22 and a3=0.08)\u003c/p\u003e","description":"","filename":"image17.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/30238f103388cd5a6dfa1118.png"},{"id":50103857,"identity":"03545f64-24e9-4e32-9f90-031c621ae52c","added_by":"auto","created_at":"2024-01-24 15:28:46","extension":"png","order_by":18,"title":"Figure 18","display":"","copyAsset":false,"role":"figure","size":22341,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of Users 1, 2 and 3\u003c/p\u003e\n\u003cp\u003eNOMA/ACO-OFDM (a1=0.7, a2=0.22 and a3=0.08)\u003c/p\u003e","description":"","filename":"image18.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/edc89e33147d2b195b9295d7.png"},{"id":50103853,"identity":"7a4e1190-afc5-47be-83bd-e94a5dfa554c","added_by":"auto","created_at":"2024-01-24 15:28:46","extension":"png","order_by":19,"title":"Figure 19","display":"","copyAsset":false,"role":"figure","size":21513,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of Users1, 2 and 3\u003c/p\u003e\n\u003cp\u003eNOMA/DCO-OFDM (a1=0.75, a2=0.2 and a3=0.05)\u003c/p\u003e","description":"","filename":"image19.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/237aee62f897b7180eef44a6.png"},{"id":50104894,"identity":"6ec1ac63-e6c9-415d-9da0-7e29b063d9db","added_by":"auto","created_at":"2024-01-24 15:36:46","extension":"png","order_by":20,"title":"Figure 20","display":"","copyAsset":false,"role":"figure","size":21642,"visible":true,"origin":"","legend":"\u003cp\u003eThe BER of Users1, 2 and 3\u003c/p\u003e\n\u003cp\u003eNOMA/ACO-OFDM (a1=0.75, a2=0.2 and a3=0.05)\u003c/p\u003e","description":"","filename":"image20.png","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/2ca4f605a5d308dd9f7dbcaf.png"},{"id":50105971,"identity":"9856b974-b42a-4697-bc4c-0e5e692b1f20","added_by":"auto","created_at":"2024-01-24 15:52:49","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1021063,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3879685/v1/9c6cbcc6-6c6c-4d2c-9add-04d325d35ad5.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"A Hybrid NOMA/OFDM Model for Next Generation Communication Systems","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAn optical technique named light fidelity, or Li-Fi, is primarily based on photodiodes (PD) [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Li-Fi offers faster transmission speeds, higher bandwidth, and the ability to operate in environments that are vulnerable to electromagnetic interference [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. This is because photodiodes (PDs), or Light Emitting Diodes (LEDs), consume very little energy. In contrast to Wi-Fi, Li-Fi does not create electromagnetic interference [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Visible light communication (VLC) recently has been proposed as a viable solution for indoor wireless communication because of its wider bandwidth, which does not violate the RF spectrum, its efficient energy, and its ability to provide ubiquitous communication [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. VLC presents significant promise for improving wireless internet access in the future. VLC is an acronym for optical wireless communication (OWC) utilizing the visible light spectrum ranging from 380\u0026ndash;780 nanometers as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eEach user in an orthogonal multiple access (OMA) system, such as orthogonal frequency division multiplexing (OFDM), is assigned an orthogonal frequency [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. By using OFDM, high-speed data transfer over a dispersive channel is made possible and the inter-symbol interference (ISI) can be eliminated by adding a small redundancy, namely the cyclic prefix (CP) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Nevertheless, commercial OWC do not employ OFDM. This is because only unipolar signals may be conveyed in VLC systems that employ intensity modulation (IM), but OFDM signals are bipolar [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. In this paper, two types of unipolar OFDM have been employed. They are asymmetrically clipped optical OFDM (ACO-OFDM) from and DC-biased optical OFDM (DCO-OFDM) from [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. The bipolar signal of ACO-OFDM is clipped to zero while in DCO-OFDM, a DC bias is added to remove the negative values. Non-orthogonal multiple access (NOMA) was suggested as a potential radio access method for 5G cellular networks [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. NOMA enhances the spectral efficiency of the system by enabling multiple users to utilize all the available frequency and temporal resources simultaneously [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In essence, NOMA differs from other multiple access methods that offer subscribers orthogonal access in terms of time, frequency, code, or space. In NOMA, all users work simultaneously in the same band, differentiated only by their power levels [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, K users can share the entire BW of OMA system whereas in NOMA, the entire transmitted power is distributed among the K users.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe transmitter side of the NOMA system uses superposition coding, and the receiver employes successive interference cancellation (SIC) for distinguishing users in the uplink and downlink [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Quadrature amplitude modulation (QAM), one of the higher-order modulation techniques, can be applied to significantly improve the spectral efficiency of NOMA based VLC systems. Since indoor VLC system uses intensity modulation/direct detection (IM/DD), QAM signal cannot directly be applied to VLC since it produces bipolar complex-valued symbols. To overcome this limitation, we could make use of O-OFDM technology, which generates real-valued signals in the time domain by utilizing the concept of Hermitian symmetry [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. A NOMA system for indoor VLC environment under realistic channel conditions is implemented in [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] and the results revel the superior performance over orthogonal frequency division multiple access (OFDMA). In [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], an ACO-OFDM NOMA model was presented in order to compute the BER analytically and through simulation vs. the SNR. A user with a higher power allocation has a lower bit error rate (BER) than one with a lower power allocation. In this work, hybrid NOMA ACO-OFDM and NOMA DCO-OFDM are proposed to serve three users located in a VLC cell at different distances away from a single transmitter (LED) situated on the center of the ceiling. Each user is assigned different power allocation conditions. The BER of the three users is estimated as a function of the SNR.\u003c/p\u003e"},{"header":"2. System Model","content":"\u003cp\u003eIn this paper, a VLC cell that serves 3 users by a single LED is assumed. The LED is located on the ceiling center of a room. The users are located varying distances apart from the transmitter. A downlink channel model for line of sight (LOS) is taken into consideration. Shadowing and reflections are not taken into account.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Channel Model for VLC Indoor Environment\u003c/h2\u003e \u003cp\u003eThe channel model shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e is considered. The point-to-point VLC system is expanded in this model to support \u003cem\u003eK\u003c/em\u003e users. \u003cem\u003eL\u003c/em\u003e is the vertical distance, in meters, between the LED and the receiving plane, while \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({r}_{k}\\)\u003c/span\u003e\u003c/span\u003e represents the horizontal separation from the LED. The angles of incidence and irradiance are \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\psi }_{k}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\varphi }_{k}\\)\u003c/span\u003e\u003c/span\u003e respectively [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Assume that all users have been ordered according to their channel quality [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${h}_{1}\u0026lt;{h}_{2}\u0026lt;\\dots \u0026lt;{h}_{k}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe power allocation coefficient is inversely proportional to the channel gain \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{k}\\propto \\frac{1}{{h}_{k}}\\)\u003c/span\u003e\u003c/span\u003e [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$${a}_{1}\u0026gt;{a}_{2}\u0026gt;\\dots \u0026gt;{a}_{k}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({h}_{k}\\)\u003c/span\u003e\u003c/span\u003e represents the channel gain of LOS link between the LED and the \u003cem\u003ek\u003c/em\u003e\u003csup\u003e\u003cem\u003eth\u003c/em\u003e\u003c/sup\u003e user [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$${h}_{k}=\\frac{\\left(m+1\\right)A {R}_{PD}}{2\\pi {d}_{k}^{2}}{cos}^{m}\\left({\\varphi }_{k}\\right){ g}_{f}\\left({\\psi }_{k}\\right){ g}_{c}\\left({\\psi }_{k}\\right)cos\\left({\\psi }_{k}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere A stands for the PD's detection area, R\u003csub\u003ePD\u003c/sub\u003e for the PD's responsivity, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({ g}_{f}\\left({\\psi }_{k}\\right)\\)\u003c/span\u003e\u003c/span\u003efor the optical filter's gain at the receiver. The distance between the \u003cem\u003ek\u003c/em\u003e\u003csup\u003e\u003cem\u003eth\u003c/em\u003e\u003c/sup\u003e user and the LED is \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({d}_{k}=\\sqrt{{r}_{k}^{2}+{L}^{2}}\\)\u003c/span\u003e\u003c/span\u003e; \u003cem\u003em\u003c/em\u003e and\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({ g}_{c}\\left({\\psi }_{k}\\right)\\)\u003c/span\u003e\u003c/span\u003e denote the Lambertian emission order of LED and optical concentrator's gain respectively, are given by [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]:\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$$m=-\\frac{1}{{log}_{2}\\left[cos\\left({\\phi }_{1/2}\\right)\\right]}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$${ g}_{c}\\left({\\varphi }_{k}\\right)=\\left\\{\\begin{array}{cc}\\frac{{n}^{2}}{{sin}^{2}\\left({\\psi }_{FOV}\\right)}\u0026amp; 0\\le {\\varphi }_{k}\\le {\\psi }_{FOV} \\\\ 0\u0026amp; {\\varphi }_{k}\u0026gt;{\\psi }_{FOV}\\end{array} \\right.$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eIn the equations \u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and \u003cspan refid=\"Equ4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\phi }_{1/2}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\psi }_{FOV}\\)\u003c/span\u003e\u003c/span\u003e and \u003cem\u003en\u003c/em\u003e denote the LED's half-power semi-angle, the receiver's angle field of view (FOV) and the refractive index constant respectively. The channel gain can be written as follows, assuming the PDs are continuously pointed toward the ceiling:\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$${h}_{k}=\\frac{\\left(m+1\\right)A {R}_{PD}}{2\\pi }\\frac{{L}^{m+1}}{{\\left({r}_{k}^{2}+{L}^{2}\\right)}^{\\frac{m+3}{2}}}{ g}_{f}\\left({\\psi }_{k}\\right){ g}_{c}\\left({\\psi }_{k}\\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 NOMA-OFDM MODEL IN VLC SYSTEM\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e depicts the entire block diagram of the suggested system model. The data of each user is modulated using M-QAM modulator. A power allocation strategy is applied to the three users.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe users have been assigned to different positions within the VLC cell, with user 1 being the farthest and user 3 being the closest, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. in which \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({d}_{1}\u0026gt;{d}_{2}\u0026gt;{d}_{3}\\)\u003c/span\u003e\u003c/span\u003e. The channel gains of the users are \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({h}_{1}\u0026gt;{h}_{2}\u0026gt;{h}_{3}\\)\u003c/span\u003e\u003c/span\u003e according to the position of each user. The power allocation factors of the three users are \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{2}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{3}\\)\u003c/span\u003e\u003c/span\u003e so that \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}\u0026gt;{a}_{2}\u0026gt;{a}_{3}\\)\u003c/span\u003e\u003c/span\u003e. Lower power should be provided to the user who has higher channel gain, and high power has to be delivered to the user with lower channel condition [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCombining the signals of all users gives [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]:\u003cdiv id=\"Equ7\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ7\" name=\"EquationSource\"\u003e\n$$X=\\sum _{k=1}^{K}\\sqrt{{a}_{k}{P}_{t}}{s}_{k}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eand for three users:\u003cdiv id=\"Equ8\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ8\" name=\"EquationSource\"\u003e\n$$X=\\sum _{k=1}^{3}\\sqrt{{a}_{k}{P}_{t}}{s}_{k}={P}_{t}(\\sqrt{{a}_{1}}{s}_{1}+\\sqrt{{a}_{2}}{s}_{2}+\\sqrt{{a}_{3}}{s}_{3})$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e8\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe signal \u003cem\u003eX\u003c/em\u003e obtained in Eq.\u0026nbsp;\u003cspan refid=\"Equ8\" class=\"InternalRef\"\u003e8\u003c/span\u003e is complex-valued which cannot be applied to the LED because the LED has to be supplied by real-valued and positive signal. Hermitian symmetry can be utilized before the inverse fast Fourier transform (IFFT) to obtain real-valued set of data symbols [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({X}_{0}\\)\u003c/span\u003e\u003c/span\u003e and the middle \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({X}_{N/2}\\)\u003c/span\u003e\u003c/span\u003e subcarriers are set to be zero [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003cdiv id=\"Equ9\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ9\" name=\"EquationSource\"\u003e\n$${X}_{H\\_DCO}=\\left[0, {X}_{1},{X}_{2}\\dots {X}_{\\frac{N}{2}-1},0, {X}_{\\frac{N}{2}-1}^{*},\\dots {,X}_{2}^{*},{X}_{1}^{*}\\right]$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e9\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ10\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ10\" name=\"EquationSource\"\u003e\n$${X}_{H\\_ACO}=\\left[0, {X}_{1},0,{X}_{3}\\dots {X}_{\\frac{N}{2}-1},0, {X}_{\\frac{N}{2}-1}^{*},0,\\dots ,{X}_{3}^{*},0, {X}_{1}^{*}\\right]$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e10\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThen, the output of the IFFT is the time-domain real-valued signal \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({x}_{n}\\)\u003c/span\u003e\u003c/span\u003e[20]:\u003cdiv id=\"Equ11\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ11\" name=\"EquationSource\"\u003e\n$${x}_{n}=\\frac{1}{N}\\sum _{k=0}^{N-1}{X}_{k} {e}^{j\\frac{2\\pi nk}{N}} 0\u0026lt;n\u0026lt;N-1$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e11\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003eN\u003c/em\u003e is the FFT/IFFT size, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({X}_{k}\\)\u003c/span\u003e\u003c/span\u003e is the \u003cem\u003ek\u003c/em\u003e\u003csup\u003e\u003cem\u003eth\u003c/em\u003e\u003c/sup\u003e subcarrier symbol.\u003c/p\u003e \u003cp\u003eA parallel to serial (P/S) converter converts the IFFT output to serial form, and then a cyclic prefix (CP) of a particular length is then added for avoiding inter symbol interference (ISI) by transforming the linear convolution into a circular convolution. A direct current (DC) is added in the case of DCO-OFDM and zero clipping of negative values when ACO-ODDM is used to gain a unipolar real-valued signal to supply the LED. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e illustrates the DCO-OFDM and ACO-OFDM modulators.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt the receiver side, the received signal at each PD (user) is [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]:\u003cdiv id=\"Equ12\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ12\" name=\"EquationSource\"\u003e\n$${y}_{k}\\left(t\\right)={h}_{k}x\\left(t\\right)+{n}_{k}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e12\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({n}_{k}\\)\u003c/span\u003e\u003c/span\u003e denotes the additive white Gaussian noise while \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({h}_{k}\\)\u003c/span\u003e\u003c/span\u003e represents the channel response between the LED and the \u003cem\u003ek\u003c/em\u003e\u003csup\u003e\u003cem\u003eth\u003c/em\u003e\u003c/sup\u003e user.\u003c/p\u003e \u003cp\u003eThe optical signal detected by the PDs is converted into an electrical form. The PD's output signal is filtered before being digitally transformed. An S/P circuit is used to convert the signal from serial to parallel after the CP has been removed. Following that, the FFT is utilized to obtain a frequency-domain output from the received time-domain signal [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003cdiv id=\"Equ13\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ13\" name=\"EquationSource\"\u003e\n$${X}_{k}=\\sum _{n=0}^{N-1}{x}_{n} {e}^{-j\\frac{2\\pi nk}{N}} 0\u0026lt;k\u0026lt;N-1$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e13\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eUser 1, with the highest transmission power, recovers its data directly without SIC process because it views the signals of the other two users as noise. User 2 employs SIC to eliminate interference from user 1. In order to detect its data, user 3 can eliminate the interference due to user 1\u0026rsquo;s and user 2\u0026rsquo;s signals via employing SIC process twice. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e explains the decoding process of the three users. The flowchart of the proposed model is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe BER can be defined as the ratio of the received corrupted bits to the total number of transmitted bits [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]:\u003cdiv id=\"Equ14\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ14\" name=\"EquationSource\"\u003e\n$$BER=\\frac{recieved corrupted bits}{Total nomber of bits}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e14\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eIn this paper, the BER of the three users is estimated as a function of the SNR with equitable constellation order (M1\u0026thinsp;=\u0026thinsp;M2\u0026thinsp;=\u0026thinsp;M3\u0026thinsp;=\u0026thinsp;4). Figures\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003e, \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e and \u003cspan class=\"InternalRef\"\u003e13\u003c/span\u003e show the BER for NOMA/DCO-OFDM model while Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e, \u003cspan class=\"InternalRef\"\u003e12\u003c/span\u003e and 14 for NOMA/ACO-OFDM model. The three users share the total transmitted power of the LED inequitably. Three cases were chosen to distribute power among users. The first case the power allocation coefficients are \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}=0.65\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{2}=0.25\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{3}=0.1\\)\u003c/span\u003e\u003c/span\u003e for user1, 2 and 3 respectively. The BER of the three users is very high for the two models. The performance has got better when the power allocation coefficients are \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}=0.7\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{2}=0.22\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{3}=0.08\\)\u003c/span\u003e\u003c/span\u003e. The lower BER is obtained when \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}=0.75\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{2}=0.2\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{3}=0.05\\)\u003c/span\u003e\u003c/span\u003e. In order to compare the performance of the three users, the BER of the three users is plotted as shown in Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e15\u003c/span\u003e, \u003cspan class=\"InternalRef\"\u003e17\u003c/span\u003e, 19 for NOMA/DCO-OFDM and Figs.\u0026nbsp;16, 18, 20 for NOMA/ACO-OFDM. Figures\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e15\u003c/span\u003e and 16 shows high BER for the two models for power allocation coefficients \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}=0.65\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{2}=0.25\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{3}=0.1\\)\u003c/span\u003e\u003c/span\u003e. The performance is enhanced as the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}\\)\u003c/span\u003e\u003c/span\u003e increases. The results illustrate that the users\u0026rsquo; BER converges to each other when the SNR increases. Figure\u0026nbsp;18 shows that the BER at SNR=35 dB is between 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e and 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e for NOMA/DCO-OFDM comparing with Fig.\u0026nbsp;20 for NOMA/ACO-OFDM the BER reaches 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e at SNR=34.5 dB. Also Fig.\u0026nbsp;19 shows that the BER reaches 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e at SNR=28.6 dB for NOMA/DCO-OFDM whereas for NOMA/ACO-OFDM at SNR=27.2 dB the BER reaches 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this paper, two models NOMA/DCO-OFDM and NOMA/ACO-OFDM are presented to estimate the BER as a function of the SNR of three users are located in an interior environment of a VLC system. Three strategies for power allocation coefficients are considered. The first strategy where the power allocation coefficients for the three users are \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}=0.65\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{2}=0.25\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{3}=0.1\\)\u003c/span\u003e\u003c/span\u003e shows poor system performance for the two models. A better BER is obtained for the second (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}=0.7\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{2}=0.22\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{3}=0.08\\)\u003c/span\u003e\u003c/span\u003e) and third (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{1}=0.75\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{2}=0.2\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({a}_{3}=0.05\\)\u003c/span\u003e\u003c/span\u003e) strategies. the first strategy. The results illustrate better performance of NOMA/ACO-OFDM than that of NOMA/DCO-OFDM. As the SNR grows, the BER of the three users converges.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAuthor contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHuang, T.-C.Y.W.-T., Lee, W.-B.: Visible Light Communication System Technology Review: Devices, Architectures, and Applications, Crystals, vol., 11, No.1098, Sep. 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(2022)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"optical-and-quantum-electronics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"oqel","sideBox":"Learn more about [Optical and Quantum Electronics](https://www.springer.com/journal/11082)","snPcode":"11082","submissionUrl":"https://submission.nature.com/new-submission/11082/3","title":"Optical and Quantum Electronics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Visible light communication (VLC), Orthogonal frequency division multiplexing (OFDM), Nonorthogonal multiple access (NOMA), Bit error rate (BER)","lastPublishedDoi":"10.21203/rs.3.rs-3879685/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3879685/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eVisible light communication (VLC) is an innovative optical wireless communication (OWC) technology that can provide both lighting and high-speed wireless data transmission. Another advantages of VLC system that allow for a wide range of applications are its fast data rate, reliable communication channels, and interference protection against electromagnetic (EM) waves. Light Emitting Diodes (LEDs) are used as transmitters while Photodetectors are utilized at the receiver side. The LED has to be supplied with a positive real valued signal. Orthogonal frequency division multiplexing (OFDM) uses multiple subcarriers with orthogonal frequencies to enhance the spectral efficiency of the system. There are two types of unipolar OFDM used to obtain real-valued and positive signal, they are DC-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM). A hybrid design NOMA/OFDM is introduced in this paper to be able to study the performance of the VLC system. Three users are positioned in different spots throughout an interior environment. User1 is considered as the farthest user with lower channel gain while user3 is the near user that has high channel gain. The bit error rate (BER) is estimated for the three users vs. the SNR with different power allocation coefficients. The results show that NOMA/ACO-OFDM performs better than NOMA/DCO-OFDM. Also, the BER of the three users converges to each other with the increasing of the SNR for the two models.\u003c/p\u003e","manuscriptTitle":"A Hybrid NOMA/OFDM Model for Next Generation Communication Systems","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-01-24 15:28:41","doi":"10.21203/rs.3.rs-3879685/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2024-03-18T13:05:59+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"e41833e0-6f47-4ddc-84a3-fbbce5a51f96","date":"2024-03-11T09:20:03+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"ff2ca2fa-db92-42fb-a382-686642016bc0","date":"2024-02-21T08:48:09+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"4d8747a3-c7f8-4cd3-9f40-a7e741ea7248","date":"2024-01-21T11:13:49+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-01-21T09:51:15+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-01-20T02:51:08+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-01-20T02:29:42+00:00","index":"","fulltext":""},{"type":"submitted","content":"Optical and Quantum Electronics","date":"2024-01-19T19:30:15+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"optical-and-quantum-electronics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"oqel","sideBox":"Learn more about [Optical and Quantum Electronics](https://www.springer.com/journal/11082)","snPcode":"11082","submissionUrl":"https://submission.nature.com/new-submission/11082/3","title":"Optical and Quantum Electronics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"77400c11-19e5-484c-91d6-922224bab4f3","owner":[],"postedDate":"January 24th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2024-01-24T15:28:41+00:00","versionOfRecord":[],"versionCreatedAt":"2024-01-24 15:28:41","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3879685","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3879685","identity":"rs-3879685","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}