We are IntechOpen, the world s leading publisher of Open Access books Built by scientists, for scientists. International authors and editors

Transcription

1 We are ItechOpe, the world s leadig publisher of Ope Access books Built by scietists, for scietists 3, , M Ope access books available Iteratioal authors ad editors Dowloads Our authors are amog the 154 Coutries delivered to TOP 1% most cited scietists 12.2% Cotributors from top 500 uiversities Selectio of our books idexed i the Book Citatio Idex i Web of Sciece Core Collectio (BKCI) Iterested i publishig with us? Cotact book.departmet@itechope.com Numbers displayed above are based o latest data collected. For more iformatio visit

2 Chapter 4 Idex Modulatio-Aided OFDM for Visible Light Commuicatios Qi Wag, Tiaqi Mao ad Zhaocheg Wag Additioal iformatio is available at the ed of the chapter Abstract Idex modulatio-aided orthogoal frequecy-divisio multiplexig(im-ofdm) is a promisig modulatio techique to achieve high spectral ad eergy efficiecy. I this chapter, the covetioal optical OFDM schemes are firstly reviewed, followed by the priciples of IM-OFDM. The applicatio of IM-OFDM i visible light commuicatio (VLC) systems is itroduced, ad its performace is compared with covetioal optical OFDM, which verifies its superiority. Fially, the challeges ad opportuities of IM-OFDM are discussed for the VLC applicatios. Keywords: idex modulatio (IM), orthogoal frequecy-divisio multiplexig (OFDM), visible light commuicatios (VLCs), spectral efficiecy, eergy efficiecy 1. Itroductio Orthogoal frequecy-divisio multiplexig (OFDM) has become a ubiquitous digital commuicatio techique, which is widely employed i visible light commuicatio (VLC) [1]. Sice itesity modulatio with direct detectio is utilized i VLC for low-cost implemetatio, the sigals modulated o the light-emittig diodes (LEDs) should be real-valued ad oegative [2]. Therefore, various optical OFDM schemes have bee proposed to satisfy these costraits, amely DC-biased optical OFDM (DCO-OFDM) [3], asymmetrically clipped optical OFDM (ACO-OFDM) [4], pulse-amplitude-modulated discrete multi-toe (PAM-DMT) [5], uipolar OFDM (U-OFDM) [6], ad Flip OFDM [7]. I all these optical OFDM schemes, Hermitia symmetry is utilized o the OFDM subcarriers before iverse fast Fourier trasform (IFFT), so that the time-domai sigals are real-valued. To esure the oegativity, DC bias ca be imposed o the resultat sigals, leadig to low-eergy efficiecy. Alteratively, special arragemets o the sigals i the time or frequecy domai are used i Refs. [3 7] without 2017 The Author(s). Licesee ITech. This chapter is distributed uder the terms of the Creative Commos Attributio Licese ( which permits urestricted use, distributio, ad reproductio i ay medium, provided the origial work is properly cited.

3 72 Visible Light Commuicatios the eed of DC bias, which geerate o-egative sigals at the cost of spectral efficiecy loss. I order to overcome the spectral efficiecy loss while maitaiig high-eergy efficiecy, several hybrid schemes are proposed [8 11]. For example, iasymmetrically clipped DCbiased optical OFDM (ADO-OFDM) [8] ad hybrid ACO-OFDM (HACO-OFDM) [9], ACO- OFDM with odd subcarriers is combied with DCO-OFDM ad PAM-DMT modulatig eve subcarriers for simultaeous trasmissio, respectively. I layered ACO-OFDM (LACO- OFDM) [10] ad ehaced U-OFDM (eu-ofdm) [11], multiple streams of ACO-OFDM or U- OFDM are superposed for higher spectral efficiecy. Recetly, the idex modulatio (IM) techique has bee itroduced to OFDM to ehace its performace [12, 13], where the iformatio is trasmitted ot oly with the classic amplitude ad phase modulatio schemes but also implicitly by the idices of the activated subcarriers. Compared with classical OFDM, IM-aided OFDM (IM-OFDM) offers a promisig trade-off betwee the bit error rate (BER) performace ad the spectral efficiecy by chagig the umber of activated subcarriers, the usage of costellatio alphabets, ad so o. I fact, the primary cocept of IM-aided OFDM was proposed two decades ago, which was termed as parallel combiatory OFDM [14]. After the developmet of spatial modulatio (SM) (or IM) i recet years, it attracts extesive attetios. I Refs. [15], subcarrier idex modulatio (SIM-OFDM) is proposed, where the status of each subcarrier (ON or OFF) carries oe-bit iformatio while the activated subcarriers are modulated by covetioal costellatios such as quadrature amplitude modulatio (QAM) ad phase shift keyig (PSK). Additioal bits ca be trasmitted by the idices of activated subcarriers, ad the eergy efficiecy is improved with iactivated subcarriers. However, sice the bit rate is ustable for differet iformatio bits, it may cause error propagatio at the receiver. To address this issue, ehaced SIM-OFDM (ESIM-OFDM) is proposed i Refs. [16], where every two subcarriers are paired ad oly oe subcarrier is activated i each pair. However, the spectral efficiecy is reduced sice the bits trasmitted by idices of subcarriers are halved. I Refs. [17], OFDM with idex modulatio (OFDM-IM) is proposed where subcarriers are partitioed ito several subblocks ad the iformatio bits are trasmitted by both the idices of activated subcarriers ad sigal costellatios i each subblock. The beefit of OFDM-IM is that less power is required sice oly a fractio of the subcarriers is employed for modulatio, while the idices of activated subcarriers ca be utilized to trasmit extra bits. However, the iactivated subcarriers waste eormous precious frequecy resources ad reduce the spectral efficiecy sigificatly. Moreover, it is show that although OFDM-IM outperforms covetioal OFDM with low spectral efficiecy below 2 bit/s/hz, it may perform eve worse tha OFDM whe high-order costellatios are utilized to achieve high spectral efficiecy [18]. Furthermore, a subcarrier-level iterleavig techique is itroduced to OFDM-IM i Refs. [19], which elarges the Euclidea distaces betwee differet trasmitted symbols. I Refs. [20], OFDM-IM is combied with space-time block codes with coordiate iterleavig, ehacig its BER performace due to the additioal diversity gai. Besides, geeralized schemes of OFDM-IM have bee proposed i Refs. [21, 22], where the umber of activated subcarriers of each OFDM subblock is variable, ad idex modulatio is performed o both the i-phase ad quadrature compoets of the modulated symbols, respectively. The geeralized schemes are capable of ehacig the spectral efficiecy of covetioal OFDM-IM sigificatly. Additioally, [23] facilitates a spectrally efficiet IM-OFDM scheme by employig various costellatios ad umbers of activated subcarriers i differet subblocks. Moreover, OFDM-IM is also combied

4 Idex Modulatio-Aided OFDM for Visible Light Commuicatios 73 with multiple-iput-multiple-output (MIMO) systems [24, 25], yieldig cosiderable performace improvemet over covetioal MIMO-OFDM. Furthermore, OFDM-IM has bee applied to uderwater acoustic commuicatios as well as vehicle-to-vehicle ad vehicle-toifrastructure applicatios [26 28], leadig to performace gais. Recetly, the dual-mode idex modulatio-aided OFDM (DM-OFDM) is proposed i Refs. [29], where all the subcarriers are utilized to carry iformatio ulike OFDM-IM. For each subblock, the subcarriers are divided ito two groups modulated by two differet costellatio modes. The idices of either group ca be used to trasmit extra iformatio bits. Therefore, DM-OFDM achieves higher spectral efficiecy tha both OFDM-IM ad covetioal OFDM. The geeralized scheme of DM-OFDM is ivoked i Refs. [30] to further ehace the spectral efficiecy, where the umber of subcarriers modulated by the same costellatio mode is alterable. I additio, the performace trade-off of IM-OFDM schemes is ivestigated with theoretical aalysis i Refs. [31, 32], which help to ehace the overall performace of IM-OFDM. Due to the distict advatages of IM-OFDM, it is also applied to VLC systems. Cosiderig the itesity modulatio property of VLC, the existet IM-OFDM schemes caot be directly itroduced to VLC, ad several optical IM-OFDM (O-IM-OFDM) schemes [33, 34] have bee proposed. Hece, O-IM-OFDM is ivestigated i this chapter by itroducig the trasceiver desig, performig the theoretical aalysis, evaluatig the umerical results, ad poitig out its challeges ad potetials. The rest of this chapter is orgaized as follows. Sectio 2 reviews the optical OFDM schemes for VLC, while the priciples of IM-OFDM for VLC are detailed i Sectio 3. I Sectio 4, several challeges ad opportuities are discussed for the deploymet of IM-OFDM i VLC, ad coclusios are draw i Sectio Optical OFDM for visible light commuicatios Due to the itesity modulatio property, the trasmitted sigals are costraied to be realvalued ad oegative i VLC. Typically, Hermitia symmetry is imposed o the frequecydomai subcarriers to geerate real outputs after the IFFT, ad the symbols modulated oto the subcarriers satisfy X k ¼ X N k,k¼1, 2,,N=2 1, ð1þ where N is the umber of subcarriers i OFDM. Moreover, X 0 ad X N/2 are set to zero for the same purpose. After the IFFT, the resultat time-domai sigal ca be formulated as x ¼ 1 pffiffiffiffi XN 1 X k exp j 2π N N k, ¼ 0, 1,,N 1, ð2þ k¼0 which is bipolar, ad various techiques have bee proposed to obtai oegative waveforms for trasmissio. Whe N 64, the distributio of x is approximately Gaussia with zero mea. I the followig, DCO-OFDM, ACO-OFDM, PAM-DMT, U-OFDM, ad hybrid optical OFDM schemes are itroduced, ad the diagram of these schemes is illustrated i Figure 1.

5 74 Visible Light Commuicatios Figure 1. Diagram of optical OFDM schemes for VLC DCO-OFDM A atural way to obtai the uipolar sigal is addig a DC bias to the bipolar sigal while clippig the remaiig egative sigal at zero. We deote the DC bias as x DC, which is usually proportioal to the square root of the electric power of x i Eq. (2). Sice the expectatio of x is zero, the optical power of the trasmitted sigal is proportioal to the DC bias whe clippig distortio is eglected. A large value of x leads to small clippig distortio, which is beeficial to the BER performace at the receiver. However, DC bias does ot carry useful iformatio, which is ot eergy efficiet. Therefore, a trade-off betwee clippig distortio ad eergy efficiecy should be made [35] ACO-OFDM I order to avoid DC bias to achieve higher eergy efficiecy, ACO-OFDM is proposed, which leaves the eve subcarriers umodulated to obtai a ati-symmetric waveform. For each positive time-domai sigal, there is a egative sigal with the same absolute value at certai positio. Specifically, the time-domai sigals of ACO-OFDM after the IFFT satisfy [4] x ACO, ¼ x ACO, þn=2, ¼ 0, 1,,N=2 1, ð3þ whose egative part ca be directly clipped at zero (asymmetrically clipped) without iformatio loss. Therefore, the trasmitted sigal of ACO-OFDM is writte as x c ACO, ¼ x ACO, þ i ACO, ¼ xaco, x ACO, 0; 0, x ACO, < 0 ð4þ For =0,1,, N 1, where i ACO, is the egative clippig distortio of ACO-OFDM. Iterestigly, the FFT of i ACO, deoted as I ACO,k oly falls o the eve subcarriers, which does ot iterfere with the useful iformatio. Therefore, a simple FFT ca be used at the receiver for detectio.

6 Idex Modulatio-Aided OFDM for Visible Light Commuicatios PAM-DMT PAM-DMT is also a asymmetrically clipped method to geerate oegative OFDM sigals. Ulike ACO-OFDM, the imagiary part of all subcarriers is utilized for data trasmissio, while the real part is left uused to produce a ati-symmetric waveform. For the imagiary part of each subcarrier, PAM costellatios are employed. The time-domai sigals of PAM-DMT after the IFFT satisfy x PAM, ¼ x PAM,N, ¼ 1,,N=2 1, ð5þ ad the trasmitted sigal of PAM-DMT deoted as x c PAM, ca be obtaied similar to Eq. (4), while we deote the egative clippig distortio of PAM-DMT as i PAM,. The FFT of i PAM, represeted by i PAM,k oly falls o the real part of each subcarrier, which does ot iterfere with the useful iformatio similar to ACO-OFDM. Therefore, a simple FFT ca be employed for detectio at the receiver U-OFDM I U-OFDM, the sigal i Eq. (2) is utilized to geerate oegative sigal. Ulike DCO-OFDM where the DC bias is used to make the sigal uipolar, the OFDM frame is separated ito two frames with the same legth. I the first frame, all the positive sigals remai the same, while the egative sigals are clipped at zero. I the secod frame, all the egative sigals are replaced by their absolute values, while the positive sigals are set to zero. At the receiver, the secod frame is subtracted from the first frame to recover the origial sigal. Afterward, the FFT ca be used to the resultat sigal for detectio similar to DCO-OFDM Hybrid optical OFDM It is oted that, although ACO-OFDM, PAM-DMT, ad U-OFDM do ot require DC bias, oly half of the resources (i either frequecy domai or time domai) are employed compared with DCO-OFDM. Therefore, their performaces are better tha DCO-OFDM oly whe low-order costellatios are used [8]. Whe high spectral efficiecy is required, DCO-OFDM is more preferred sice all the resources are used. Recetly, some hybrid optical OFDM schemes have bee proposed to achieve better trade-off betwee spectral efficiecy ad eergy efficiecy. I ADO-OFDM, the ACO-OFDM sigal is superposed by the DCO-OFDM sigal, where oly the eve subcarriers are modulated by DCO-OFDM to avoid the iterferece [8]. I this way, all the subcarriers are utilized for modulatio, leadig to improved spectral efficiecy. At the receiver, the symbols o the odd subcarriers for ACO-OFDM are firstly demodulated after the FFT. I order to detect the symbols o the eve subcarriers for DCO-OFDM, the clippig distortio of ACO-OFDM should be elimiated firstly, which is estimated by the recovered symbols of ACO-OFDM. However, DC bias is still required i ADO-OFDM (although the power is reduced), which is iefficiet i terms of power. I HACO-OFDM, ACO-OFDM is combied with PAM-DMT whose eve subcarriers are modulated by PAM [10]. The odd subcarriers i PAM-DMT are also left uused as i ADO-OFDM.

7 76 Visible Light Commuicatios Therefore, it is very similar to ADO-OFDM at both the trasmitter ad the receiver. Sice PAM- DMT does ot require DC bias, HACO-OFDM is more eergy-efficiet compared with ADO- OFDM. However, the real part of eve subcarriers is umodulated, thus its spectral efficiecy is still limited. To fully utilize the frequecy resources, LACO-OFDM is proposed i Ref. [9], where multiple layers of ACO-OFDM are combied for simultaeous trasmissio. Differet layers employ differet subcarriers to geerate the oegative ACO-OFDM sigals, while successive iterferece cacellatio is used at the receiver to recover all the symbols i each layer. Compared with covetioal ACO-OFDM, the spectral efficiecy of LACO-OFDM is approximately doubled with the same costellatio order. A similar method is applied to U-OFDM called eu-ofdm, which combies differet depths of U-OFDM to further improve the spectral efficiecy [11]. I differet depths, various repetitios are required so that they ca be recovered at the receiver. 3. Idex modulatio-aided OFDM for visible light commuicatios I this sectio, two represetative IM-OFDM schemes, amely OFDM-IM ad DM-OFDM, will be ivestigated i terms of their priciples ad applicatios i VLC Priciples of IM-OFDM The basic idea of OFDM-IM is to divide the subcarriers ito several groups ad the idices of activated subcarriers i each group ca be used to covey extra iformatio. We deote the umber of subcarriers i OFDM as N, while m bits are trasmitted withi oe OFDM symbol. The m bits are split ito g groups each cosistig of p bits, where we have p = m / g. Besides, the subcarriers are divided ito g subblocks as well, ad each subblock has = N / g subcarriers. I each subblock of OFDM-IM, k out of subcarriers are activated for modulatio, while the others are left empty. Therefore, the idices of the activated subcarriers ca carry p 1 bits idex iformatio, which is give by p 1 ¼ log 2 k, ð6þ where deotes the iteger floor operator. Whe M ary costellatiomis used for the k-activated subcarriers, p 2 bits ca be trasmitted by each OFDM subblock, which is give by p 2 ¼ klog 2 ðmþ: ð7þ Therefore, the umber of total trasmitted bits i a OFDM-IM frame is expressed as m ¼ gðp 1 þ p 2 Þ¼g log 2 þ klog k 2 ðmþ, ð8þ ad the spectral efficiecy is give by

8 Idex Modulatio-Aided OFDM for Visible Light Commuicatios 77 γ IM ¼ m log 2 þ klog N ¼ k 2 ðmþ bit=s=hz: ð9þ Take ¼ 4, k¼ 2, M¼ 2, for example. The spectral efficiecy of OFDM-IM is 1 bit/s/hz, which is idetical to the spectral efficiecy of covetioal OFDM scheme with BPSK modulatio. Sice oly half of the subcarriers are activated i OFDM-IM, the eergy efficiecy is improved sigificatly. It is show i Eq. (6) that the umber of idex bits i OFDM-IM is costat whe ad k are fixed. Whe high-order costellatios are used i activated symbols, the iformatio bits provided by the idex patters are egligible for the whole spectral efficiecy. The spectral efficiecy loss of the iactivated subcarriers caot be compesated by the idex bits. Moreover, frequecy resource is very precious, which is ufavorable to be wasted. Therefore, DM- OFDM is proposed i Ref. [29] to fully exploit the subcarriers ad idex iformatio. I DM-OFDM, two costellatio setsm A adm B are utilized for each subblock, whose sizes are M A ad M B, respectively. I order to esure the detectio at the receiver, the two costellatios sets should have o commo costellatios poits, that is,m A M B ¼. I each subblock, the subcarriers are divided ito two groups A ad B, whose idex sets are I A ad I B. The symbols im A adm B are used for modulatio i subcarrier groups A ad B, respectively. Sice I B ca be easily determied by I A, we deote I A as the idex patter of the OFDM subblock, which ca be utilized to covey idex bits. Whe k subcarriers are modulated bym A i each subblock, the umber of total trasmitted bits of a DM-OFDM symbol is expressed as m ¼ g log 2 k þ klog 2 ðm A Þþð kþlog 2 ðm B Þ, ð10þ ad the spectral efficiecy of DM-OFDM is calculated as γ DM ¼ m log 2 þ klog N ¼ k 2 ðm A Þþð kþlog 2 ðm B Þ 3.2. OFDM-IM for visible light commuicatios bit=s=hz: ð11þ OFDM-IM ca be applied to VLC with some modificatios. The block diagram of OFDM-IMbased VLC system [33] is illustrated i Figure 2. Sice itesity modulatio is utilized i VLC, Hermitia symmetry is required to geerate real-valued sigals. Therefore, oly half of the subcarriers are utilized for groupig ad bit mappig, while the other half of the subcarriers ca be obtaied by simple Hermitia symmetry. After N-poit IFFT, the bipolar sigals are passed through the uipolar coversio block to geerate uipolar sigals for LED emissio. Whe DC-biased optical OFDM-IM (DCO-OFDM-IM) is cosidered, a DC bias is utilized, while the remaiig egative sigals are directly clipped at zero. Whe uipolar OFDM-IM (U-OFDM-IM) is used, the OFDM frame is separated ito two frames with the same legth

9 78 Visible Light Commuicatios Figure 2. Block diagram of OFDM-IM-based VLC system. similar to U-OFDM. All the positive sigals are trasmitted i the first frame, while the egative sigals are iverted for trasmissio i the secod frame. Therefore, the spectral efficiecies of DCO-OFDM-IM ad U-OFDM-IM are give by log 2 þ klog k 2 ðmþ γ DCO IM bit=s=hz; 2 log 2 þ klog k 2 ðmþ γ U IM ¼ bit=s=hz: 4 ð12þ ð13þ At the receiver, if U-OFDM-IM is employed, the secod frame is subtracted from the first frame. Afterward, a N-poit FFT is performed o the time-domai sigals. Ulike covetioal optical OFDM where the detectio ca be performed symbol by symbol, the optical OFDM-IM requires subblock-by-subblock detectio sice idices of activated subcarriers are ukow at the receiver. Without loss of geerality, we cosider the symbols i oe subblock, where the received symbols after the FFT ca be writte as R i ¼ H i X i þ W i,i¼ 1, 2,,, ð14þ where H i, X i, ad W i deote the chael respose, trasmitted symbol, ad additive white Gaussia oise (AWGN) with the variace of N 0 i the frequecy domai, respectively. Whe all the symbols are cosidered i the subblock, Eq. (14) ca be rewritte i the vector form as R ¼ HX þ W, ð15þ where the diagoal matrix H ¼ diagðh 1,H 2,,H g Þ. The optimal receiver employs maximum likelihood (ML) detectio, which cosiders all the possible subblock realizatios with differet activated subcarrier idices ad the costellatio poits by miimizig the metric as follows:

10 Idex Modulatio-Aided OFDM for Visible Light Commuicatios 79 f^i, ^Xg ¼argmi I, X kr HXk2, ð16þ where I is the idex patter of the activated subcarriers. The ML detector should cosider log 2 þklog 2 k 2 ðmþ possible realizatios, whose complexity is very high for large values of, k, ad M. Therefore, low-complexity detectio is required for practical implemetatio. I order to reduce the complexity of the receiver, log-likelihood ratio (LLR) detector is proposed by calculatig the logarithm of the ratio betwee a posteriori probabilities of the frequecy-domai symbols beig either ozero or zero. A larger LLR meas it is more likely that the correspodig subcarrier is activated. The LLR for the i-th subcarrier is give by 0 ηðiþ ¼log X M PrðX i ¼ S t jr i Þ PrðX i ¼ 0jR i Þ C A, 1 i, t¼1 1 ð17þ where S t M.Sice X M t¼1 PrðX i ¼ S t Þ¼k= ad PrðX t ¼ 0Þ ¼ð kþ=, the LLR ca be rewritte as ηðiþ ¼lðkÞ lð kþþ jr ij 2 N 0 þl XM t¼1 exp 1! jr i H i S t j 2 : ð18þ N 0 Whe the activated subcarriers are detected, the symbols o the subcarriers ca be demodulated idepedetly as the covetioal optical OFDM. Therefore, the complexity of the LLR detector for optical OFDM-IM is similar to that of covetioal optical OFDM DM-OFDM for visible light commuicatios The block diagram of DM-OFDM-based VLC system [34] is show i Figure 3. Ulike other IM-OFDM schemes where several subcarriers are empty i each subblock, DM-OFDM utilizes two distiguishable costellatio sets to modulate all the subcarriers, thus achievig higher spectral efficiecy. The spectral efficiecies of DCO-DM-OFDM ad U-DM-OFDM are give by log 2 þ klog k 2 ðm A Þþð kþlog 2 ðm B Þ γ DCO DM bit=s=hz: 2 log 2 þ klog k 2 ðm A Þþð kþlog 2 ðm B Þ γ U DM ¼ bit=s=hz: 4 ð19þ ð20þ

11 80 Visible Light Commuicatios Figure 3. Block diagram of DM-OFDM for VLC systems. Figure 4. A example of DM-OFDM costellatio desig form A adm B with M A = 8 ad M B =4. Obviously, it is crucial to fid the good combiatios of two costellatio sets i DM-OFDM. To esure good BER performace, the miimum Euclidea distace betwee the two costellatios should be equal to that of the poits withi each costellatio. Therefore, oe ca firstly desig a costellatio with M A þ M B poits ad the separate the poits ito two costellatios.

12 Idex Modulatio-Aided OFDM for Visible Light Commuicatios 81 Specifically, whe M A ¼ M B ¼ M, we ca employ the poits i the 2M ary QAM costellatio. I Figure 4, a example of DM-OFDM costellatio desig is provided form A adm B with M A ¼ 8adM B ¼ 4. The detectio of DM-OFDM is similar to that of OFDM-IM i a subblock-by-subblock maer. Whe ML detectio is used, we have f^i A, ^Xg ¼argmi I A, X kr HXk2, ð21þ which still has high complexity. The LLR detector for DM-OFDM is give by M B k ηðiþ¼l þl XM A exp 1! jr i H i S A t M A ð kþ N j2 l XM B exp 1! jr i H i S B t t¼1 0 N j2, ð22þ 0 t¼1 where S A t M A ad S B t M B. Differet from OFDM-IM, Eq. (22) calculates the logarithm of the ratio betwee the a posteriori probabilities of the frequecy-domai symbols beig modulated by mapper A ad mapper B. Whe the idex patter is detected, the symbol o each subcarrier ca be demodulated by the correspodig demapper Performace evaluatio The BER performaces of OFDM-IM ad DM-OFDM schemes are compared with their covetioal optical OFDM couterparts i VLC systems. Ulike ateas i the radio frequecy commuicatio systems, LEDs are used i VLC systems, which have oliear trasfer characteristics, leadig to distortios o the trasmitted sigal beyod the liear rage. The oliearity ca be simply modeled as 8 < V mi,x< V mi ; TðxÞ ¼ x, V mi x V max ; ð23þ : V max,x> V max, where V max ad V mi deote the maximum ad miimum allowed amplitudes, respectively. I the simulatios, the size of IFFT is set to 256, ad the umber of subcarriers i each subblock is 4. I OFDM-IM, 2 subcarriers are activated i each subblock. While i DM-OFDM, two subcarriers are modulated by mapper A, ad the other two subcarriers are modulated by mapper B. The liear rage of LEDs is [0, 1], ad the DC bias is set to 0.5 for DCO-OFDM, DCO-OFDM-IM, ad DCO-DM-OFDM. I U-OFDM, U-OFDM-IM, ad U-DM-OFDM, o DC bias is required. Figure 5 illustrates the BER performaces of OFDM-IM ad the covetioal optical OFDM schemes, where quadrature phase shift keyig (QPSK) ad biary phase shift keyig (BPSK) are utilized i OFDM-IM ad its covetioal couterparts, respectively. I the simulatios, the x-axis E b /N 0 stads for the sigal-to-oise ratio per bit. Besides, the iput eergies ito

13 82 Visible Light Commuicatios Figure 5. Performace compariso betwee OFDM-IM ad the covetioal optical OFDM schemes. Figure 6. Performace compariso betwee OFDM-IM, DM-OFDM, ad the covetioal optical OFDM schemes at higher spectral efficiecy.

14 Idex Modulatio-Aided OFDM for Visible Light Commuicatios 83 LEDs are set as 13 ad 13.5 dbm (ot icludig the DC eergy) for DC-based ad U-based schemes, respectively. For both cases, OFDM-IM achieves about ad bit/s/hz spectral efficiecy gais over the covetioal DCO-OFDM ad U-OFDM. However, it ca be see that OFDM-IM still outperforms its covetioal couterparts at the BER level of 10 3,siceehaced spectral efficiecy leads to reduced average bit eergy E b, yieldig smaller required E b /N 0. Whe higher spectral efficiecy is cosidered, OFDM-IM might perform eve worse tha covetioal optical OFDM. Figure 6 shows the BER performaces of OFDM-IM, DM-OFDM ad the covetioal optical OFDM with a spectral efficiecy of bit/s/hz for DC-based schemes ad bit/s/hz for U-based schemes, where the iput eergies ito LEDs are set as 13 ad 13.5 dbm (ot icludig the DC eergy) for DC-based ad U-based schemes, respectively. I covetioal optical OFDM schemes, 8 QAM is employed o each subcarrier, while 32 QAM is used i OFDM-IM. I DM-OFDM, the costellatios utilized are show i Figure 4. From Figure 6, it is show that OFDM-IM suffers from performace loss compared with covetioal optical OFDM with high spectral efficiecy, while DM-OFDM achieves more tha 1 ad 2 db performace gais over U-OFDM ad DCO-OFDM at the BER of 10 3, for the reaso that all the subcarriers are used for modulatio, ad the idices of subcarriers ca provide additioal dimesio for trasmissio. 4. Challeges, opportuities, ad future research treds 4.1. Dimmig compatibility I VLC systems, illumiatio is a importat fuctio of LEDs. Therefore, the modulatio scheme should be compatible with dimmig cotrol. Whe the required illumiatio level is chaged, the data rate should ot fluctuate too much. I additio, the modulatio scheme should support extreme illumiatio requiremets such as very low or very high itesities. Several schemes have bee proposed for covetioal optical OFDM to support dimmig cotrol i VLC [36, 37], which ca be exteded to the IM-OFDM schemes. Furthermore, give the property of IM-OFDM, we ca use differet umber of activated subcarriers for various illumiatio requiremets MIMO trasmissio I idoor eviromets, multiple LEDs are istalled to provide sufficiet illumiatio. Therefore, it is worthwhile to study the modulatio schemes i the MIMO VLC systems [38]. For MIMO trasmissio, low-complexity trasceiver should be desiged uder the itesity modulatio costrait. Moreover, the idex modulatio ca be utilized i the time domai ad space domai, ad multi-dimesioal idex modulatio i time, frequecy, ad space domais might be useful to achieve better performace [39, 40] Performace improvemet Curret researches o IM-OFDM for VLC oly cosider simple combiatios of optical OFDM ad IM. Hybrid optical OFDM schemes itroduced i Sectio 2.5 ca be cosidered to further

15 84 Visible Light Commuicatios improve the spectral efficiecy of IM-OFDM for VLC systems. At the trasmitter, IM-OFDM for VLC suffers from high peak-to-average power ratio (PAPR), causig oliear distortios of LEDs. Thus, efficiet PAPR reductio techiques have to be employed to combat with the oliearity of LEDs. Besides, the low-complexity ad high-performace receiver desig also requires attetio for the deploymet of IM-OFDM i low-cost VLC systems. 5. Coclusios I this chapter, to shed light o the developmet of IM-OFDM schemes i VLC, we itroduce the priciples of optical OFDM ad IM-OFDM, which are exemplified by several represetative schemes. It is idicated that various IM-OFDM schemes are capable of ehacig the eergy efficiecy or the spectral efficiecy compared with covetioal OFDM, leadig to improved BER performace. Theoretical aalysis ad umerical results demostrate that these IM-OFDM schemes ca ehace the overall performace compared with its covetioal couterparts. Therefore, IM-OFDM would be a promisig modulatio techique for future VLC systems. Moreover, the challeges ad opportuities are discussed for the deploymet of IM-OFDM i VLC systems, which are beeficial to researchers who are iterested i this field. Author details Qi Wag 1, Tiaqi Mao 2 ad Zhaocheg Wag 2 * *Address all correspodece to: zcwag@tsighua.edu.c 1 School of Electroics ad Computer Sciece, Uiversity of Southampto, Uited Kigdom 2 Departmet of Electroic Egieerig, Tsighua Natioal Laboratory for Iformatio Sciece ad Techology (TNList), Tsighua Uiversity, Beijig, Chia Refereces [1] Tsoev D, Chu H, Rajbhadari S, McKedry JJD, Gu E, Haji M, et al. A 3-Gb/s sigle- LED OFDM-based wireless VLC lik usig a gallium itride μled. IEEE Photoics Techology Letters. 2014;26(7): [2] Armstrog J. OFDM for optical commuicatios. Joural of Lightwave Techology. 2009;27(3): [3] Carruthers J ad Kah J. Multiple subcarrier modulatio for odirected wireless ifrared commuicatio. IEEE Joural Selected Areas i Commuicatios. 1996;14(3):

16 Idex Modulatio-Aided OFDM for Visible Light Commuicatios 85 [4] Armstrog J ad Lowery A. Power efficiet optical OFDM. Electroic Letters. 2006;42 (6): [5] Lee S, Radel S, Breyer F, ad Kooe A. PAM-DMT for itesity-modulated ad directdetectio optical commuicatios. IEEE Phtoics Techology Letters. 2009;21(23): [6] Tsoev D, Siaovic S, ad Haas H. Novel uipolar orthogoal frequecy divisio multiplexig (U OFDM) for optical wireless. I: Proc. IEEE Veh. Tech. Cof.; May 2012; Yohohama, Japa. pp. 1 5 [7] Ferado N, Hog Y, ad Viterbo E. Flip-OFDM for uipolar commuicatio systems. IEEE Trascatios o Commuicatios. 2012;60(12): [8] Dissaayake S ad Armstrog J. Compariso of ACO-OFDM, DCO-OFDM ad ADO- OFDM i IM/DD systems. Joural of Lightwave. Techology. 2013;31(7): [9] Wag Q, Qia C, Guo X, Wag Z, Cuigham D, ad White I. Layered ACO-OFDM for itesity-modulated direct-detectio optical wireless trasmissio. Optics Express. 2015;23(9): [10] Rajha B ad Kavehrad M. Hybrid asymmetrically clipped OFDM-based IM/DD optical wireless system. Joural of Optical Commuicatios ad Networkig. 2014;6(4): [11] Tsoev D ad Haas H. Avoidig spectral efficiecy loss i uipolar OFDM for optical wireless commuicatio. I: Proc. IEEE ICC; Ju. 2014; Sydey, Australia. pp [12] We M, Cheg X, ad Yag L. Idex Modulatio for 5G Wireless Commuicatios. Spriger; p [13] Basar E. Idex modulatio techiques for 5G wireless etworks. IEEE Commuicatios Magazie. 2016;54(7): [14] Freger P ad Svesso N. Parallel combiatory OFDM sigallig. IEEE Trasactios o Commuicatios. 1999;47(4): [15] Abu-Alhiga R ad Haas H. Subcarrier-idex modulatio OFDM. I: Proc. 20th IEEE It. Symp. Idoor Mobile Radio Commu.; Sep. 2009; Tokyo, Japa. pp [16] Tsoev D, Siaovic S, ad Haas H. Ehaced subcarrier idex modulatio (SIM) OFDM. I: Proc. IEEE GLOBECOM Workshops; Dec. 2011; TX, USA. pp [17] Basar E, Aygolu U, Paayirci E, ad Poor H. Orthogoal frequecy divisio multiplexig with idex modulatio. IEEE Trasactios o Sigal Processig. 2013;61(22): [18] Ishikawa N, Sugiura S, ad Hazo L. Subcarrier-idex modulatio aided OFDM-Will it work?. IEEE Access. 2016;4: [19] Xiao Y, Wag S, Da L, Lei X, Yag P, ad Xiag W. OFDM with iterleaved subcarrieridex modulatio. IEEE Commuicatio Letters. 2014;18(8):

17 86 Visible Light Commuicatios [20] Basar E. OFDM with idex modulatio usig coordiate iterleavig. IEEE Wireless Commuicatios Letters. 2015;4(4): [21] Fa R, Yu Y, ad Gua Y. Geeralizatio of orthogoal frequecy divisio multiplexig with idex modulatio. IEEE Trasactios o Wireless Commuicatio. 2015;14 (10): [22] Zheg B, Che F, We M, Ji F, Yu H, ad Liu Y. Low-complexity ML detector ad performace aalysis for OFDM with i-phase/quadrature idex modulatio. IEEE Commuicatio Letters. 2015;19(11): [23] Yag X, Zhag Z, Fu P, ad Zhag J. Spectrum-efficiet idex modulatio with improved costellatio mappig. I: Proc. IEEE HMWC; Oct. 2015; Xi a, Chia. pp [24] Basar E. Multiple-iput multiple-output OFDM with idex modulatio. IEEE Sigal Processig Letters. 2015;22(12): [25] Basar E. O multiple-iput multiple-output OFDM with idex modulatio for ext geeratio wireless etworks. IEEE Trasactios o Sigal Processig. 2016;64(15): [26] We M, Cheg X, Yag L, Li Y, Cheg X, ad Ji F. Idex modulated OFDM for uderwater acoustic commuicatios. IEEE Commuicatios Magazie. 2016;54(5): [27] We M, Li Y, Cheg X, ad Yag L. Idex modulated OFDM with ICI self-cacellatio i uderwater acoustic commuicatios. I: Proc. IEEE Asilomar Cof. Sigals, Syst, Comput.; Nov. 2014; Pacific Grove, CA, USA. pp [28] Cheg X, We M, Yag L, ad Li Y. Idex modulated OFDM with iterleaved groupig for V2X commuicatios. I: Proc. IEEE It. Cof. Itell. Trasp. Syst.; Oct. 2014; Qigdao, Chia. pp [29] Mao T, Wag Z, Wag Q, Che S, Hazo L. Dual-mode idex modulatio aided OFDM. IEEE Access. 2017;5:50 60 [30], Mao T, Wag Q, ad Wag Z. Geeralized dual-mode idex modulatio aided OFDM. IEEE Commuicatio Letters. 2017;21(4): [31] We M, Cheg X, ad Yag L. Optimizig the eergy efficiecy of OFDM with idex modulatio. I: Proc. IEEE It. Cof. Commu. Syst.; Nov. 2014; Macau, Chia. pp [32] Li W, Zhao H, Zhag C, Zhao L, ad Wag R. Geeralized selectig sub-carrier modulatio scheme i OFDM system. I: Proc. IEEE ICC Workshops; Ju. 2014; Sydey, NSW, Australia. pp [33] Basar E, Paayirci E. Optical OFDM with idex modulatio for visible light commuicatios. I: Proc. IEEE It. Workshops o Opt. Wirel. Commu.; Sep. 2015; Istabul, Turkey. pp [34] Mao T, Jiag R, ad Bai R. Optical dual-mode idex modulatio aided OFDM for visible light commuicatios. Optics Commuicatios. 2017;391:37 41

18 Idex Modulatio-Aided OFDM for Visible Light Commuicatios 87 [35] Wag Z, Wag Q, Che S, ad Hazo L. A adaptive scalig ad biasig scheme for OFDM-based visible light commuicatio systems. Optics Express. 2014;22(10): [36] Wag Q, Wag Z, ad Dai L. Asymmetrical hybrid optical OFDM for visible light commuicatios with dimmig cotrol. IEEE Photoics Techology Letters. 2015;27 (9): [37] Wag Q, Wag Z, Dai L, ad Qua J. Dimmable visible light commuicatios based o multilayer ACO-OFDM. IEEE Photoics Joural. 2016;8(3) [38] Wag Q, Wag Z, ad Dai L. Multiuser MIMO-OFDM for visible light commuicatios. IEEE Photoics Joural. 2015;7(6) [39] Basar E, Paayirci E, Uysal M, ad Haas H. Geeralized LED idex modulatio optical OFDM for MIMO visible light commuicatios. I: IEEE ICC 2016; May 2016; Kuala Lumpur, Malaysia [40] Sugiura S, Che S, ad Hazo L. Geeralized space-time shift keyig desiged for flexible diversity-, multiplexig- ad complexity-tradeoffs. IEEE Trasactios o Wireless Commuicatios. 2011;10(4):

19