###### Abstract  
Optical wireless communication is an energy-efficient and cost-effective solution for high-speed and highly-secure wireless connections. In this paper, we compare, discuss, and analyze three popular optical orthogonal frequency division multiplexing (OFDM) techniques, such as DC-biased optical OFDM (DCO-OFDM), asymmetrically-clipped optical OFDM (ACO-OFDM), and unipolar OFDM (U-OFDM), for underwater optical wireless communication systems. The peak power constraint, bandwidth limit of the light source, turbulence fading underwater channel, and the channel estimation error are taken into account. To maximize the achievable data propagation distance, we propose to optimize the modulation index that controls the signal magnitude, and a bitloading algorithm is applied. This optimization process trades off the clipping distortion caused by the peak power constraint and the signal to noise ratio (SNR). The SNR and clipping effects of the three compared OFDM techniques are modeled in this paper. From the numerical results, DCO-OFDM outperforms ACO- and U-OFDM when the transmitted bit rate is high compared to the channel bandwidth. Otherwise, U-OFDM can provide a longer propagation distance or requires less transmitted power. optical wireless communications; underwater communications; OFDM; peak power constraint; propagation distance; BER; band-limited channel; turbulence fading +
Footnote †: journal:  
0000-0001-9991-4600-0000-0000-0000-0000-00000-00000-00000-00000-00000-00000-00000-00000-00000-000000-00000-000000-00000-000000-000000  
UOWC is an emerging technique for underwater systems with a high-speed transmission for moderate distance communication. Although UOWC can provide a longer propagation distance than underwater RF systems, an extremely long-range underwater communication should currently rely on acoustic systems. Therefore, in UOWC, most of the research focuses on improving the transmission data rate and propagation distance [5; 6; 7]. A light-emitting diode (LED) working as a high power-efficient light source has attracted more attention recently and shows a big potential in underwater optical wireless communication systems [8; 9; 10; 11]. Single-carrier frequency division multiple access (SC-FDMA) can achieve a lower peak to average power ratio than OFDM, which has the potential to be beneficial for underwater optical wireless communications as well [12; 13; 14; 15]. Since LED is a band-limited device, multi-carrier modulation is more suitable for the LED-based UOWC systems. Orthogonal frequency division multiplexing (OFDM) is the key technique for 4G wireless communications, which is also widely used in many applications. Due to its resistance to inter-symbol interference (ISI) and high spectral efficiency, OFDM is a candidate technique for optical wireless communications. In OFDM techniques, \(M\)-ary quadrature amplitude modulation (M-QAM) is commonly used for a high efficiency of bandwidth. Unlike the conventional RF OFDM, the OFDM for the optical wireless communication systems can only transmit real and non-negative signals. Nowadays, optical OFDM has attacked much attention, both in academia and industry [16; 17; 18]. Recently, there have been many popular optical OFDM techniques such as DC-biased optical OFDM (DCO-OFDM), asymmetrically-clipped optical OFDM (ACO-OFDM), and unipolar OFDM (U-OFDM) [19; 20; 21]. To avoid the negative signals, DCO-OFDM adds a DC bias to the original bipolar real-valued OFDM signals. However, since the DC-biased signals cannot totally get rid of negative-valued information, the signals under zero must be hard clipped. ACO-OFDM and U-OFDM are recently-proposed optical OFDM techniques for intensity modulation and direct detection systems [20]. For ACO-OFDM, only odd frequency subcarriers are used for modulating data, and even frequency subcarriers are set to zero. By that, the zero clipping effects can only introduce interference on the even subcarriers [22].