 Open Access
 Authors : Manal Ali, Hamada Esmaiel
 Paper ID : IJERTV11IS010184
 Volume & Issue : Volume 11, Issue 01 (January 2022)
 Published (First Online): 09022022
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Symmetric PAMDMT for Spectrum and Energy Efficient Lifi Systems
Manal Ali
Electrical Engineering Department, Faculty of Engineering, Aswan University,
Aswan 81542, Egypt
Hamada Esmaiel
Electrical Engineering Department, Faculty of Engineering, Aswan University, Aswan 81542, Egypt
Department of Information and Communication, School of Informatics, Xiamen University, Xiamen 316005, China
Abstract A novel digital modulation technique based on orthogonal frequency division multiplexing (OFDM) is proposed for light fidelity (LiFi). The proposed technique is applicable to all OFDMbased intensity modulated and direct detected (IMM/DD) systems. DC biased optical OFDM (DCOOFDM) is widely used in highspeed LiFi systems. However, the high peak toaveragepowerratio (PAPR) of OFDM signals requires a high DC bias, which makes it energy inefficient for mobile communications. The proposed technique builds on the principles of pulse amplitude modulation (PAMDMT) but improves the design by doubling the spectral efficiency and reducing the computational complexity. The proposed scheme offers an energyefficient alternative to unipolar OFDM schemes at high spectra efficiencies since the gain of the scheme increases with the increased modulation size. For example, a gain of 2.75 dB is achieved over PAMDMT at a spectral efficiency of 1 bits/s/Hz, whereas a significant 20 dB gain is achieved when our proposed technique is compared with PAMDMT at a spectral efficiency of 5 bits/s/Hz. This is ideal for uplink communication in LiFi since the mobile terminal battery capacity is limited, thanks to the powerhungry mobile applications.
Keywords Light fidelity (LiFi), Optical orthogonal frequency division multiplexing (OFDM), pulse amplitude modulation discrete multitone (PAMDMT)

INTRODUCTION
The demand for highspeed wireless access has increased significantly. Global mobile data traffic is estimated to grow by a factor of 4.5 to reach 226 Exabyte per month in 2026 [1]. The growth of mobile data traffic is driven by the improved mobile devices capabilities and dataintensive contents [1]. The radio frequency (RF) spectrum is limited and unable to sustainably support this exponential increase in traffic demand and wireless access. Alternative frequency bands are undeniably required to satisfy the worldwide demand for wireless communication. It is widely acknowledged that the optical domain is a promising candidate to support this highlydemanded indoor traffic.
Visible light communication
(VLC) is aiming at reusing the existing lighting infrastructure to enable an energyefficient telecommunication links that do not require any licensing [2]. Light fidelity (LiFi) takes the concept of peertopeer visible light links into the next level by enabling attocellular networks that support handover and mobility [34].
LiFi modulation techniques are primarily based on intensity modulation and direct detection (IM/DD). This is mainly since LiFi is expected to be enabled using offtheshelf light emitting diode (LED) and photodiode (PD), since LiFi is assumed to
reuse the available inexpensive lighting infrastructure. Therefore, advanced coherent modulation techniques cannot be realized due to the lack of coherence in the light output of an LED. Off the shelf LED can produce incoherent light, therefore, the information can be encoded in the light intensity. Pulsed modulation techniques such as pulseamplitude modulation (PAM), binary phase shift keying (BPSK), and pulse width modulation (PWM) can implemented by simply feeding the baseband signal into the LED. However, the major problem of these modulation techniques is the increased inter symbol interference (ISI) when the symbol rate increases. This is mainly driven by the low pass filter (LPF) characteristic of the conventional LED and the frequency selectivity that could arise from the nonlineofsight (LoS) optical wireless communication (OWC) channels.
Given the fact that offtheshelf LED have a low modulation bandwidth (between 320 MHz) [5]. It is of paramount importance that LiFi modulation techniques can support high spectral efficiencies. Orthogonal frequency division multiplexing (OFDM) is one of the main suitable candidates to satisfy the requirement of spectral efficiency due to its support of bit and power loading algorithms that can adapt the OWC channel utilization to the possible capacity [6]. Conventional RF domain OFDM techniques can be adopted in the optical domain. However, the optical signal output of a conventional LED can only be realvalued and unipolar. Therefore, the modulation waveform needs to be both real valued and positive. Hermitian symmetry is usually used in conventional direct current (DC) to enable a realvalued output of the OFDM modulator. Whereas a DC bias is usually added to shift the negative samples of the waveform to become positive. However, the addition of the DC bias results in an additional power dissipation that is not directly used in information transmission. Several different modulation techniques have been proposed to address the problem of spectrum efficiency in conventional DCbiased optical OFDM (DCOOFDM).
Unipolar OFDM techniques attempt to use the properties of Fourier transformation to adjust the output of the OFDM modulator. For example, pulseamplitude modulation discrete multitone modulation (PAMDMT) allows for the time domain waveform to be unipolar by loading symbols on the imaginary component of the subcarriers [7]. This allows the signal to be clipped at zero levels with a fixed power penalty of 3 dB. Therefore, it can enable data transmission over LED in LiFi. However, PAMDMT uses 1D modulation schemes such as
Fig. 1. The block diagram of the transmitter of SPAMDMT. IFFT denotes IFFT; P/S denotes a parallel to serial conversion.
PAM, whereas DCOOFDM uses 2D schemes such as QAM. Therefore, it can achieve the same spectral efficiency of DCO OFDM. However, it requires a higher signaltonoise ratio (SNR) as the bit error ratio (BER) performance of Mary quadrature amplitude modulation (MQAM) is equivalent to the BER performance of M2QAM [8]. these unipolar modulation techniques have a disadvantage that prevents their use when highspectral efficiencies are required. Asymmetrically clipped optical OFDM (ACOOFDM) restricts the data loading into the oddindexed subcarriers to obtain an asymmetry property in the timedomain output waveform [9]. ACOOFDM uses only half of the subcarriers, therefore the spectral efficiency of ACOOFDM is half of that in DCO OFDM.
In this paper, we propose a modification for PAMDMT that allows it to cut the waveform size in half, and therefore, allows for higher spectral efficiency. The proposal is based on building asymmetry in the timedomain waveform so that only half of the OFDM frame is required to be transmitted. This is achieved in the frequency domain by loading the data into the real component of the subcarriers. Therefore, our proposed modulation technique can transmit twice the information of that in PAMDMT which improves the spectral efficiency of unipolar OFDM modulation techniques for LiFi systems. The paper starts with investigating timedomain symmetry properties that can enable improvements in the energy and spectral efficiencies. A new technique is proposed based on creating a timedomain symmetry. Afterwards, we investigated the DCbias requirements of the newly proposed technique. This is achieved by performing MonteCarlo simulation study on all the possible biasing points. The optimal DC bias points are then selected and used in a differet MonteCarlo simulation study to investigate the BER performance of the proposed technique and compare it to DCOOFDM and PAM DMT.
This paper is organized as follows, Section 2 briefly introduce the PAMDMT modulation techniques, Section 3 discuss the proposed modulation technique symmetrical pulse amplitude modulation discrete multitone modulation (SPAM DMT) and explains its block diagram and basic properties. Section 4 presents the simulation results and discusses the main findings, and finally, Section 5 concludes the paper.

PAMDMT
The generation of PAMDMT starts with a conventional OFDM modulator, as shown in Fig.1 and Fig.2. However, the modulation format of PAM is chosen instead of QAM. This is
because PAMDMT loads data on the imaginary component of the frequency subcarriers X[k]=jB[k], where B[k] is the M PAM symbol. An inverse fast Fourier transform (IFFT) is applied on the frequency domain data symbols and the result is then called the timedomain PAMDMT waveform. A Hermitian symmetry is applied on the input PAM frequency domain symbols to restrict the timeddomain output to be real valued. This is done as follows:
(1)
where k=1, 2, …, NFFT1, X[0]=0, and X[NFFT/2]=0. Similar to other OFDM techniques, the last NCP symbols are appended at the start of each PAMDMT frame to ensure that singletap equalizer can be used to equalize the communication channel. The timedomain PAMDMT waveform can be given as follows:
(2)
Based on Hermitian symmetry, this can be give as
follows:
(3)
The antisymmetry of PAMDMT can be shown in (1), as follows xPAM[n]=xPAM[Nn], as shown in Fig.3(a). The positive samples from the first half of waveform are repeated as negative samples in a reverse order at the second half of the waveform, whereas the negative samples from the first half of waveform are repeated as positive samples in a reverse order at the second half. This means that the clipping distortion has a Hermitian symmetry and therefore, falls only on the real valued subcarriers. Therefore, clipping the timedomain at zerolevel does not distort the data symbols on the imaginary component of the subcarriers. However, a 3 dB SNR penalty affects the demodulator due to clipping the timedomain PAM DMT signal at zerolevel. The spectral efficiency of PAM DMT modulation scheme can be given as follows:
(4)
This is exactly similar to the spectral efficiency of DCO OFDM. However, the BER performance of MPAMDMT is equivalent to the BER performance of M2QAM DCOOFDM. Therefore, the energy efficiency advantage of PAMDMT quickly disappears as the constellation size increase.
Fig. 2. The block diagram of the receiver of SPAMDMT. CP denotes cyclic prefix; S/P denotes a serial to parallel conversion. FFT denotes.
Fig. 3. (a) Timedomain frame of a PAMDMT modulator showing antisymmetry property. (b) Time domain frame of a SPAMDMT modulator showing symmetry property
Therefore, a solution is needed to achieve energy efficiency at high spectral efficiencies. The concept of SPAMDMT is like the concept of PAMDMT. However, instead of loading the QAM symbols on the imaginary components of the subcarriers, the data symbols are loaded on the real components of the subcarriers, X[k]=A[k], where A[k] is the MPAM symbol. As a result, the timedomain waveform would have a Hermitian symmetry that is different from the Hermitian symmetry applied on the incoming frequency domain data symbols. This is essential because both the time domain and frequencydomain signals are realvalued.
The generation of a timedomain SPAMDMT waveform starts with a PAM modulator as shown in Fig.1. The data are loaded on the realvalued subcarriers X[k]=A[k], where A[k] is the MPAM symbol, and then an IFFT operation is applied on the incoming data symbols. The timedomain SPAMDMT waveform can be given as follows:
(5)
Based on Hermitian symmetry, this can be given as follows:
(6)
Therefore, based on (2) it can be observed that xSPAM[n]=xSPAM[Nn] as shown in Fig.3(b). The positive samples from the first half of waveform are repeated as positive samples in a reverse order at the second half of the waveform,
Fig. 4. The BER performance of SPAMDMT as a function of SNR and DC bias for 4PAMDMT
whereas the negative samples from the first half of waveform are repeated as negative samples in a reverse order at the second half. This means that the signal is repeated halfway, and it is only sufficient to send half of the timedomain frame to retrieve the data symbols at the receiver, shown in Fig.2. The last N/2 are removed, and the whole operation is repeated twice before a cyclic prefix is added at the start of the waveform. However, the timedomain waveform cannot be clipped at zerolevel, as the clipping distortion would distort the data carrying symbols and impair the successful data recovery. Therefore, a DC bias is required as it is the only method to convert the bipolar SPAMDMT waveform into a unipolar, similar to DCOOFDM. However, the spectral efficiency of SPAMDMT is twice of that in DCOOFDM, as given below:
(7)
Therefore, it is possible to use a lower constellation size to achieve the same spectral efficiency of DCOOFDM. For example, 2SPAMDMT can achieve the same spectral efficiency as 4QAM DCOOFDM. This is expected to allow for a better BER performance of SPAMDMT as will be shown later in Section III.
The receiver starts by removing the cyclic prefix and then duplicating half of the received timedomain frame by creating a timedomain Hermitian symmetric frame. An FFT is applied on the frame and equalization is achieved using a singletab frequency domain equalizer. The selection of the DC bias for each of the constellation sizes is performed by trying all the possible DC bias points and comparing the required SNR at BER values below 1Ã—104. The threshold of the BER was chosen so that it allows for forward error correction (FEC) to work. Fig.4 and Fig.5 shows the results for the DC bias selection for 4SPAMDMT and 16SPAMDMT, respectively. An optimal DC bias point for 4SPAMDMT and 16SPAM
Fig. 5. The BER performance of SPAMDMT as a function of SNR and DC bias for 16PAMDMT
DMT were selected at 7.8 dB and 11.2 dB, respectively. The optimal DC bias points for the rest of the constellation sizes are shown in Table.I.
TABLE I. THE DC BIAS POINTS FOR SPAMDMT
Modulation order
Spectral efficiency [bits/s/Hz]
Bias
[dB]2PAM
1
5.3
4PAM
2
7.8
8PAM
3
9.4
16PAM
4
11.2
32PAM
5
12

NUMERICAL RESULTS
A numerical study was conducted to examine the performance of the proposed modulation techniques compared with DCOOFDM and PAMDMT at the same spectral efficiency. The comparison is based on an additive white Gaussian noise (AWGN) channel with zerolevel and upper clipping to model an ideal optical frontend design of a LiFi channel. A LPF is assumed to model the frequency profile of the LED similar to many studies [10].
The performance of 2SPAMDMT is compared with the performance of 4PAMDMT and 4QAM DCOOFDM at spectral efficiency of 1 bits/s/Hz in Fig.6. It is shown that 2 SPAMDMT outperform both 4PAMDMT and 4QAM DCOOFDM by 2.75 dB and 3 dB at a BER of 104 respectively. It was shown in Fig.7 that the SNR performance of SPAMDMT outperforms that of PAMDMT and DCO OFDM by 20.3 dB and 3.6 dB, respectively. This is a huge gain in energy efficiency that could be claimed against all unipolar OFDM modulation techniques that are similar to PAMDMT, such as DCOOFDM and Flip OFDM [11]. The SNR gain achieved by the proposed technique against DCO OFDM and PAMDMT is presented in Table II for the rest of the spectral efficiency values.
Fig. 6. BER performance comparison of SPAMDMT, PAMDMT and DCOOFDM at a spectral efficiency of 1 bits/s/Hz as a function of the SNR.
Fig. 7. BER performance comparison of SPAMDMT, PAMDMT and DCOOFDM at a spectral efficiency of 5 bits/s/Hz as a function of the SNR.
TABLE II. THE GAIN OF THE PROPOSED SPAMDMT IN COMPARISON WITH DCOOFDM AND PAMDMT
Spectral efficiency
Gain [dB]
DCOOFDM
PAMDMT
1
3
2.75
2
2.85
5.7
3
3
10.5
4
2.75
15.1
5
3.6
20.3
In addition to the energy efficiency gains that SPAMDMT can offer against DCOOFDM and PAMDMT, there are additional gains from a computational complexity point of view. Given the fact that SPAMDMT uses a 1D modulation format which allows for a simplified IFFT and FFT operations. Furthermore, it is also possible to make use of the fact that only half of the OFDM frame is required to be transmitted, the IFFT computational complexity at the transmitter can be given as O(N/4Ã—log2(N/2)). Using the same logic, the computational
complexity at the receiver can be given as O(N/4Ã—log2(N/2)). This is because half of the frame is required at the output of the FFT operation and because 1D modulation format is being used PAM. These are compared with O(NÃ—log2(N)). and O(N/2Ã—log2(N)). for the transmitter and receiver of DCO OFDM modulation scheme.

CONCLUSIONS
A novel modulation technique is proposed in this paper to improve the spectral and energy efficiency of LiFi modulation techniques. The proposed technique introduces a different type of timedomain symmetry to PAMDMT which allows for an increase in the spectral efficiency by a factor of 2, given that one half of the signal is only required to be sent. Although, the proposed technique still requires a DC bias to be transmitted through LEDs, the value of the DC bias is much lower to that required for DCOOFDM. Moreover, SPAMDMT offers significant gains when compared with unipolar OFDM modulation techniques such as PAMDMT. A 20 dB gain is achieved at a spectral efficiency of 5 bits/s/Hz when compared with PAMDMT, and a 3.6 dB gain when compared with DCOOFDM at the same spectral efficiency. Therefore, the proposed technique requires lower SNR to operate at the same datarate as DCOOFDM and PAMDMT, which allows for a reduced power requirements and increased range of LiFi systems. In addition, the proposed scheme offers a computationally efficient solution that outperforms other conventional modulation techniques such as DCOOFDM and PAMDMT. SPAMDMT can be used for the uplink in LiFi where a computationally efficient solution is required so that the battery of the mobile terminal is preserved, and it can be used for long distance LiFi links where the received optical power is limited.
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