## 1 Introduction  
Over the last few years, optical wireless communications (OWC) have attracted more and more attention in both commercial and military society due to its wide unregulated bandwidth and insensitivity to interference from electro-magnetic waves. And the OWC played an important role in emergency communications for disaster resistance [1]. The research scope of OWC included free-space optical communications (FSO), visible light communications (VLC) and underwater optical wireless communications (UOWC). Taking available devices, cost and complexity into account, the transmission scheme in OWC primarily adopted intensity modulation and direct detection (IM/DD) except for a few coherent detections that were mainly used in long-distance FSO communications. With the simple transmitter and receiver, a single-carrier scheme (e.g., pulse amplitude modulation (PAM)) was researched in many fields. [PERSON] et al. [2] proposed a feed-forward pre-equalization together with a PAM modulation scheme to achieve the transmission of data rates >1 Gb/s in VLC. To mitigate multipath dispersion, [PERSON] et al. [3] proposed a PAM-modulated single-carrier system using frequency-domain equalization. [PERSON] et al. [4] demonstrated a high-speed UWOC link over a 12 m and 20 m underwater channel, which reached a data rate of 2 Gbps and 1.5 Gbps based on NRZ OOK modulation. [PERSON] et al. [5] reported a 16 Gbps PAM-4 UWOC system using a laser diode (LD) with 488 nm wavelength. [PERSON] et al. [6] reported a UOWC system based on the PAM scheme, which can be used in practical marine experiments. Although the transmitters and receivers using PAM had low complexity, the spectra efficiency (SE) of PAM was low, which was not suitable for OWC systems with limited modulation bandwidth, like OWC systems using LED. When SE was a driving factor, OFDM, in conjunction with IM/DD, was more popular in OWC systems. [PERSON] et al. [7] demonstrated a 26 m air-water link with an OFDM scheme and data rate of 5.5 Gbps based on a 520 nm laser diode. [PERSON] et al. [8] achieved a high-speed VLC with 9 Gbps QAM-OFDM and 450 nm GaN laser diode. [PERSON] et al. [9,10] reported an improved m-QAM-OFDM transmission and a two-path parallel OFDM scheme for OWC. [PERSON] et al. [11] proposed a non-Hermitian symmetry OFDM for multiple-input, multiple-output VLC. [PERSON] et al. [12] demonstrated a UWOC transmission with a 4.8 Gbps data rate based on a 16-QAM-OFDM-modulated 450 nm laser. Due to the high SE, a multiple-carrier scheme (e.g., OFDM), together with IM/DD, would contribute to the development of high-speed OWC over short- to medium-transmission distances. As a promising multicarrier transmission technique, FOFDM, which has been theoretically proposed for wireless applications [13], has only half of the subcarrier space of the conventional OFDM [14; 15]. It has been extensively investigated in optical fiber communications [16; 17; 18; 19]. FOFDM can reduce the complexity of the transmitter when compared to a conventional OFDM system having the same spectral efficiency and total data rate [20; 21]. For generic linear channels, in [15], a computationally efficient fast-OFDM scheme enabling simple single-tap equalization without sacrificing data rate was proposed. To evaluate the performance of FOFDM signal transmitting over a complex media channel, this work, as far as we know, experimentally investigated the performance of FOFDM scheme for OWC over a complex media channel which was comprised of a weak turbulence channel generated by an atmospheric turbulence simulator, together with a water tank, for the first time. The feasibility of this scheme was validated by the experiment results, which showed that the 4-ASK-FOFDM scheme was not sensitive to the generated weak turbulence, and its BER performance was very close to the conventional 16-QAM-OFDM scheme under the same spectral efficiency. Hence, the 4-ASK-FOFDM technique can be a promising alternative to the conventional 16-QAM-OFDM technique for optical wireless communications.