Spectral Efficient Asymmetrically Clipped Hybrid FBMC for Visible Light Communication

Filter bank multicarrier (FBMC) modulation has shown suﬃcient potential for wireless communication. A hybrid optical FBMC technique is proposed to improve the spectral eﬃciency of a visible light communication (VLC) system. In this technique, a hybrid asymmetrically clipped optical oﬀset quadrature amplitude modulation FBMC (HACO-OQAM-FBMC) modulation technique is used. Asymmetrically clipped optical FBMC (ACO-FBMC) is used for odd subcarriers, and pulse amplitude modulation-discrete multitone (PAM-DMT) is used for the even subcarriers. The proposed hybrid scheme uses an intensity modulation/direct detection (IM/DD) channel. It is shown that there is no interference on odd subcarriers using the proposed method and receiver demodulation is similar to that of ACO-FBMC receiver. However, clipping noise of ACO-FBMC falls on PAM-DMTsubcarriers, which can be cancelled at receiver processing after estimation. The analytical performance of the proposed technique is compared using parameters, namely, bit error rate (BER), spectral eﬃciency, computational complexity, and peak to average power ratio (PAPR). It is found that HACO-OQAM-FBMC is more spectral eﬃcient than ACO-FBMC and other OFDM-based techniques.


Introduction
Orthogonal frequency division multiplexing (OFDM) is used extensively in wired and wireless communication systems. Despite its popularity, it possesses some limitations also such as high out-of-band distortion and long cyclic prefix (CP). In previous years, alternative multicarrier modulation techniques have been studied to address OFDM issues. e filter bank multicarrier (FBMC) modulation technique is considered one of the most potential waveforms for next-generation wireless communications due to its robustness against intersymbol interference (ISI) [1].
Visible light communication (VLC) has drawn great attention of researchers due to its unlicensed spectrum, no health hazards, and no electromagnetic interference [2]. In radio frequency (RF) communications, FBMC has been considered as a strong competitor of OFDM [3,4]. But, for the VLC system, which uses an intensity modulation/direct detection (IM/DD) channel, the signal should be real and unipolar [5][6][7]. Moreover, the average power of RF communication is the mean square value of the signal, but the average power of VLC is the mean value of the signal. A lower average power is usually preferred for RF communication, but VLC has a predefined average intensity according to the dimming target, which is not an objective function but a constraint [8,9]. So, FBMC techniques as implemented in RF communications straightforward cannot be used in the VLC system. e modulation techniques mostly used in the VLC system are asymmetrical clipped optical OFDM (ACO-OFDM) and DC biased optical OFDM (DCO-OFDM). Spectral efficiency of ACO-OFDM is low because only odd subcarriers are used and even subcarriers are neglected to meet the requirement of the VLC system [10]. To improve the spectral efficiency of the VLC system mostly proposed techniques in literature are based on OFDM. Asymmetrically clipped DC biased OFDM (ADO-OFDM) to improve the spectral efficiency of ACO-OFDM is proposed in [11]. But, it uses DC bias for even subcarriers which is a power inefficient method. In [12], the author proposed enhanced hybrid asymmetrically clipped OFDM (EHACO-OFDM) which increases spectral efficiency but has more complexity.
Till date, there is no work reported in the literature which discusses the assessment of the hybrid FBMC modulation technique for VLC based on the IM/DD channel.
In this paper, the authors proposed hybrid asymmetrically clipped optical offset quadrature amplitude modulation FBMC (HACO-OQAM-FBMC) in which ACO-FBMC is used for the odd subcarriers and PAM-DMT is used for the even subcarriers. e clipping noise of ACO-FBMC falls only on even subcarriers. e receiver of HACO-OQAM-FBMC consists of four signals: the wanted ACO-FBMC signal, clipping noise from ACO-FBMC, PAM-DMT signal, and noise added in the channel. e clipping noise of ACO-FBMC can be accurately separated from PAM-DMT after estimation before demodulation. e authors' contributions can be summarized as follows: (1) e system model is proposed for the HACO-OQAM-FBMC modulation technique (2) Scaling techniques are used for the proposed FBMCbased model for the VLC system (3) e performance analysis is carried out using spectral efficiency, bit error rate (BER), computational complexity, and peak to average power ratio (PAPR) for the FBMC-based VLC system (4) Non-DC bias methods are used for the proposed model of the VLC system (5) e performance of ACO-OFDM, DCO-OFDM, ADO-OFDM, EHACO-OFDM, ACO-FBMC, DCO-FBMC, and HACO-OQAM-FBMC modulation techniques has been compared e content of this paper is systematized as follows. Section 2 gives the system model for the HACO-OQAM-FBMC modulation technique. Performance analysis of the proposed system models is performed in Section 3 using parameters, probability density function, bit error probability, peak to average power ratio, computational complexity, spectral efficiency, and power spectral density. e conclusion is presented in Section 4.

Proposed Modulation Techniques
System Model e system model consists of a transmitter and receiver functional diagram for the VLC system. Figure 1 shows the functional diagram of the transmitter and receiver section of the HACO-OQAM-FBMC modulation technique.
In the transmitter section of HACO-OQAM-FBMC, the offset quadrature amplitude modulation (OQAM) is used for signal mapping. Hermitian symmetry is used to fit the signal into VLC system requirements. Odd subcarriers are modulated using an ACO-FBMC modulator, and a PAM-DMT modulator is used for even subcarriers. An optical modulator is employed to convert an electrical signal to optical signal. In the receiver section of HACO-OQAM-FBMC, an optical demodulator is used to convert an optical signal back to an electrical signal. For extraction of even and odd subcarriers, ACO-FBMC and PAM-DMTdemodulators are used. e equalizer is used to remove the distortion, and finally, the signal is extracted using a decoder. e signal analysis of the HACO-OQAM-FBMC transmitter and receiver is given in the next section.

Signal Analysis of the HACO-OQAM-FBMC Transmitter.
e data subcarriers are given by e ACO-FBMC consists of odd subcarriers Z odd , and PAM-DMT consists of even subcarriers Z even . e even and odd subcarriers are sent to the PAM-DMT and ACO-FBMC modulators which consist of IFFT and clipping of the signal. e transmitted ACO-FBMC signal is represented by Similarly, the transmitted signal of PAM-DMT is given by where n afcn and n dfcn are the clipping noise of ACO-FBMC and PAM-DMT [13]. Using equations (1) and (2), the transmitted signal of HACO-OQAM-FBMC is given by Upper-case letters are used to represent frequency-domain signals, and lower-case letters are used to represent time-domain signals.

ACO-FBMC Modulator.
e OQAM modulator output shown in Figure 1 consists of a real and imaginary signal. Upsampling is performed before prototype filter to increase the sampling rate. A PHYDYAS protype filter is used in the proposed method. After IFFT execution, clipping is required to fulfil optical modulator constraints. e block diagram of the ACO-FBMC modulator is shown in Figure 2.
e PHYDYAS prototype filter [14] is given by e coefficients c i depend on the overlapping factor K, and the procedure for calculation is given in [15]. For example, when the value of the overlapping factor K � 4, the coefficients are given by

Signal Analysis of the HACO-OQAM-FBMC Receiver.
e received signal for HACO-OQAM-FBMC is given by 2 International Journal of Optics where n d awgn and n a awgn are the additive white Gaussian noise (AWGN) of the even and odd subcarriers. A flat channel is assumed for one receiver. After applying FFT to equation (6), the signal is given by e clipping noise and AWGN noise are removed using ACO-FBMC and PAM-DMT demodulators. e decoded signal is given by

ACO-FBMC and PAM-DMT Signal
Detection. e optical demodulator output, as shown in Figure 1, consist of ACO-FBMC, PAM-DMT, AWGN noise, and clipping noise. e received signal from optical demodulator is converted into frequency domain using FFT. After that, the ACO-FBMC signal is detected. To recover the PAM-DMT signal, we have to use the clipping noise estimation technique, as shown in Figure 3. In this technique, for proper detection and scaling, odd subcarriers are multiplied by 2 because power of the M-QAM symbols reduced by half due to clipping operation [16,17]. Subtraction of estimated clipping noise which falls on PAM-DMT (even) subcarriers from received ACO-FBMC symbols will give PAM-DMT symbols.

Performance Analysis of HACO-OQAM-FBMC
e performance analysis is carried out using probability density function, bit error rate, spectral efficiency, computational complexity, peak to average power ratio, and power spectral density.

Probability Density Function (PDF) of HACO-OQAM-FBMC.
e PDF of HACO-OQAM-FBMC can be calculated by convolving the PDF of ACO-FBMC and PDF of PAM-DMT. e PDF of ACO-FBMC is given by [18,19] e PDF of PAM-DMT given by [18,20] e PDF of HACO-OQAM-FBMC can be calculated using convolution of equations (9) and (10).
Substituting equations (9) and (10) in equation (11) gives International Journal of Optics where σ 2 is the variance of the signal and is given by E z 2 .
Using identity [21,22], e optical power of HACO-OQAM-FBMC is given by e electrical power of HACO-OQAM-FBMC is given by Equations (14) and (15) have no closed-form solution. erefore, they are calculated using a numerical process. e probability density function of HACO-OQAM-FBMC is given in Figure 4. It consists of both theoretical and simulated PDF and shows similar performance. e PDF curve shows that the proposed technique is power efficient.

Bit Error Probability (BEP) of HACO-OQAM-FBMC.
e bit error probability of HACO-OQAM-FBMC is given by [23] where M is the constellation size and S av /N o is the average signal to noise ratio (SNR)/symbol. Figure 5 shows the bit error rate (BER) performance of the HACO-OQAM-FBMC modulation technique. e FBMC overlapping factor (k � 4) is used for simulation. e theoretical and simulation results show almost similar performance.
As shown in Figure 6, the performance of HACO-OQAM-FBMC shows 2 dB degradation when compared with ACO-OFDM because only half of the power is allocated to odd subcarriers (ACO) in HACO-OQAM-FBMC. In reality, the performance of the proposed scheme is better when compared with ACO-OFDM because the OFDM performance is degraded due to the use of cyclic prefix (CP) [24]. e BER performance of the proposed technique is better than that of DCO-OFDM and ADO-OFDM because no DC bias is used, and hence, it is power efficient.
Analysis of Figure 7 shows that there is a 3 dB penalty for HACO-OQAM-FBMC as compared to ACO-FBMC, but actually both have similar performance because half power e BER performance of the proposed technique is power efficient as compared to that of DCO-FBMC and EHACO-OFDM because no DC bias is used in the proposed technique.

Computational Complexity of HACO-OQAM-FBMC.
e computational complexity of HACO-OQAM-FBMC is given by where RM HF is the number of real multiplications for HACO-OQAM-FBMC. M is the constellation size, N is the number of subcarriers, and K is the overlapping factor.
where RA HF is the number of real additions for HACO-OQAM-FBMC. e computational complexity of HACO-OQAM-FBMC is given in Figures 9 and 10. e computational complexity of ACO-OFDM is the lowest as compared to other techniques because of the no-DC bias technique and use of OFDM. OFDM has low computational complexity than FBMC [25]. HACO-OQAM-FBMC technique computational complexity is the highest among other techniques because extra processing is required for PAM-DMT signals.

Spectral Efficiency (SE) of HACO-OQAM-FBMC.
e spectral efficiency of HACO-OQAM-FBMC can be calculated from [4] where α is the coding rate of channel, ζ � Number of bits/Subcarrier � log 2 (M), and ] T ] F is the time-frequency efficiency. e spectral efficiency of ACO-OFDM, DCO-OFDM, ACO-FBMC, DCO-FBMC, EHACO-OFDM, and HACO-OQAM-FBMC is given in Figure 11. e spectral efficiency of HACO-OQAM-FBMC is higher than that of other modulation techniques and almost similar to that of DCO-FBMC because DCO-FBMC does not use Hermitian symmetry and, hence, has more subcarriers for data communication. All other modulation techniques have low spectral efficiency as compared to the proposed method because they are OFDMbased techniques which have low spectral efficiency as compared to FBMC [25].

Power Spectral Density (PSD) of HACO-OQAM-FBMC.
e power spectral density of HACO-OQAM-FBMC is shown in Figure 12. PSD of ACO-FBMC, DCO-FBMC, and HACO-OQAM-FBMC is the same because the same prototype filter is used. e PSD of the proposed technique is better than that of the OFDM-based technique because FBMC has low out-of-band distortion as compared to OFDM.

Conclusions
is research work proposes Hybrid FBMC schemes for visible light communication based on an intensity modulation/direct detection (IM/DD) channel. In hybrid asymmetrically clipped DC biased optical FBMC (HACO-OQAM-FBMC), ACO-FBMC is used for the odd subcarriers and for even subcarriers, PAM-DMT is used. e PAPR of the proposed scheme is 0.96 dB more than ACO-FBMC because of hybridization. e spectral efficiency of HACO-OQAM-FBMC is higher than that of ACO-OFDM, DCO-OFDM, ACO-FBMC, and EHACO-OFDM due to use of even subcarriers and the FBMC multicarrier modulation    technique. e computational complexity of HACO-OQAM-FBMC is higher because extra processing is required for even subcarriers. It is shown that there is no interference on odd subcarriers because PAM-DMT and receiver demodulation is achieved using the clipping estimation technique. Future directions can be reducing computational complexity and PAPR of HACO-OQAM-FBMC.

Data Availability
e data used to support the findings of this study in the manuscript are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.