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The area to which this thesis contributes is the design of signal transceiver techniques to expand the set of legacy RF modulation methods that can be adapted to VLC. As previously mentioned, VLC systems employing off-the- shelf LEDs as front-end devices widely rely on the IM/DD signaling method for transmitting/receiving the VL signal [98]. This signaling scheme transmits data based on the intensity of the VL carrier and restricts the transmitted signal to be real and non-negative (unipolar, and one-dimensional); i.e., the legacy RF modulation techniques that can be directly used in VLC are limited to incoherent schemes such as on-off Keying (OOK), multilevel pulse position modulation (M- PPM) and unipolar optical pulse amplitude modulation (M-OPAM). Indeed, the IEEE 802.15.7 standard [1], which was completed in April 2011, offers three different physical layer (PHY) types grouped by data rate. PHY I operates from 11.67 kb/s to 266.6 kb/s, PHY II operates from 1.25 Mb/s to 96 Mb/s, and PHY III operates from 12 Mb/s to 96 Mb/s and is dedicated to multiple optical sources using a particular modulation format called color shift keying (CSK). Whereas, the modulation formats used in PHY I and PHY II devices consist of OOK and variable PPM (VPPM) as it is illustrated in Table 1.1. However, incoherent techniques are not spectrally or energy efficient techniques when compared to coherent mapping formats in RF-based systems such as M-QAM and OFDM. Recall that the motivation for the use of multidimensional symbols such as M- QAM to increase the transmission throughput of the communication channel dates back to the early days of information theory when Shannon recognized that as the dimensionality of the signal constellation grows, the distance between signal points in the constellation increases, in turn, this provides more immunity to noise

Table 1.1: VLC IEEE 802.15.7 physical layer standard standard Modulation Speed Multi-Optical

Sources PHY I OOK, VPPM 11.67 to 266.6 kb/s No PHY II OOK, VPPM 1.25 to 96 Mb/s No

and a lower error probability. To this end, current research activities in VLC focus on converting signals generated by traditional two-dimensional (2D) symbols into one-dimensional signals suitable for transmission over the IM/DD channel. Up to date, the proposed solutions impose a special arrangement on the transmitted frame structure in the time, space or frequency domain leading to the loss of more than half of the SE performance of the system. Moreover, the applied DC-bias, which is required to account for the uni-polarity constraint of the IM/DD channel, is another factor that degrades the error performance of the VLC systems. Indeed, even in ISI-free environments, real-valued OFDM has been adapted since it can convert any complex symbols to bipolar-real-valued symbols, and therefore, it enables the utilization of modulation techniques such as M-QAM or phase-shift keying (PSK) in the context of IM/DD channel. Although real-valued OFDM is very popular in VLC systems, it suffers from energy inefficiency when it is compared to the conventional coherent techniques where two bipolar carrier signals are individually modulated. The energy inefficiency is due to the required DC shift to convert the bipolar time-domain signal of the real-valued OFDM to a positive-unipolar signal. The aforementioned technique is known as direct-current- optical-OFDM (DCO-OFDM) [5] and it has been shown in the literature that this method requires a high DC-shift (i.e., low EE) to perform the bipolar to unipolar signal conversion. This is due to the high peak to average power ratio (PAPR) characteristic of the OFDM time-domain signal. In other words, a high PAPR implies that the minimum of the negative peak of the time-domain signal is likely to be very low such that a high DC-bias would be required to shift the bipolar time- domain signal. Instead of applying a DC-shift that is equal to the lowest negative peak of the transmit signal, it is widely adapted in literature to add a multiple of the standard deviation of the original OFDM signal distribution to the transmit signal for minimizing the required DC-shift; any remaining negative parts of the transmit signal is then clipped. Though the aforementioned method reduces the applied DC-shift, clipping leads to an additional clipping noise that significantly affects the bit error rate (BER) performance of this approach, especially for high mapping format sizes [29].

To improve the EE of DCO-OFDM, schemes such as asymmetrically clipped optical-OFDM (ACO-OFDM) [5] and unipolar-OFDM (U-OFDM) [105, 115] have been proposed in the literature. Both solutions, however, reduce the BW efficiency by a factor of two when compared to DCO-OFDM because of the

restrictions imposed on their frame structures, as well as reduce the EE by 3 dB in comparsion to conventional OFDM schemes. In addition, by taking into account that larger M-QAM modulation constellations require more power, the EE improvement achieved by these solutions becomes negligible as the constellation size M increases. To address the SE loss of the U-OFDM scheme, the authors in [54] proposed an enhanced U-OFDM (eU-OFDM) scheme. Briefly, the method is said to superimpose multiple U-OFDM streams to improve the SE of U- OFDM. A major drawback of the eU-OFDM is that it requires many information streams (an infinite number in theory) to achieve the same SE as DCO-OFDM. However, practical implementation of the eU-OFDM typically limits the number of superimposed streams to three due to the computational complexity (CC) and memory requirements. To reduce further the SE gap between DCO-OFDM and eU-OFDM, the GeneRalizEd ENhancEd UnipolaR OFDM (GREENER-OFDM) was proposed in [7]. However, the GREENER-OFDM scheme introduces more complexity when compared to eU-OFDM, and it does not mitigate the CC and memory requirements of eU-OFDM.

Therefore, the primary objectives of this research work are

• Developing an EE low-complexity unipolar transmission scheme for VLC systems; high energy efficient by completely avoiding the DC-shift. Low complexity, by avoiding multi-layered signal transmission/reception in com- parison with U-OFDM or GREENER OFDM.

• Developing new transceiver schemes that are more energy and spectral effi- cient than the existing ones for transmitting symbols from 2D constellations in VLC systems. The methods should allow orthogonal transmission of the conventional multidimensional modulation techniques such as M-QAM in an EE manner by reducing the DC-shift component of the transmit VL signal, and also increases the SE by not necessarily only relying on the frequency or time dimensions as in the existing VLC systems.

• Evaluating the EE and SE performance of our proposed schemes using Monte Carlo simulations.

• Deriving the analytical error rate performance of the proposed transmission techniques to validate the simulated performance results of the proposed methods.

• Comparing the performance of the proposed transmission schemes with traditional and existing techniques such as the state of the art M-OPAM, carrierless amplitude and phase modulation (CAP), DCO-OFDM and ACO- OFDM to provide numerical case studies of the EE benefits and advantages of the proposed methods.