# A Novel OFDM-Based Time Domain Quadrature GSM for Visible Light Communication System
## 1 Introduction  
The upcoming 6G communications will have 100-1000 times higher transmission rates than 5G networks, which are expected to fulfill the emerging requirements in challenging applications, including virtual presence, industry 4.0, and personalized health services [1]. Nevertheless, the current frequency bands of radio frequency (RF) communication are limited and may not be sufficient to meet the growing demands. Therefore, exploring new frequency sources to address the spectrum scarcity issue has become very important [2]. Visible light communication (VLC) technologies have been widely considered as a promising RF-alternative option for future indoor communications due to their abundant license-free spectrum, no electromagnetic interference radiation, and simultaneous illumination and communication [3, 4]. Due to the low cost and simple modulation, light emitting diodes (LEDs) are usually employed as the optical transmitters in the VLC systems [5]. However, the bandwidth of LEDs is limited; thus, the achievable spectral efficiency (SE) of the VLC systems will be affected to a certain extent as the broadband signal is injected [6]. Various methods have been proposed to address the above issue. Advanced modulation formats that can effectively utilize the available bandwidth have been extensively investigated inVLC systems [7; 8]. In addition, the advanced transmission schemes, such as the multiple-input multiple-output (MIMO) architecture, can also be deployed in VLC to enhance the system capacity under the fixed modulation bandwidth [9]. Spatial modulation (SM) and spatial multiplexing (SMP) are types of MIMO schemes that can be used in VLC systems, which have undergone a great deal of research attention due to their negligible inter-channel interference and low implementation complexity [10; 11]. However, achieving the higher SE was still challenging since the signal transmission is only implemented through a single activated LED while the remaining are just utilized for illumination. To address this issue, researchers proposed generalized spatial modulation (GSM), which can selectively activate multiple LEDs for efficient data transmission in the spatial domain [12; 13]. In addition, as a multicarrier modulation technology, orthogonal frequency division multiplexing (OFDM) is capable of making full use of the limited bandwidth, and can be employed to combat the inter symbol-interference introduced by multiple reflections from the walls of the indoor environment. By involving the OFDM in the GSM scheme, the capacity of the VLC system can be increased consequently. In [14], two OFDM-based VLC-GSM schemes were proposed where one is a frequency domain GSM (FD-GSM) and the other is a time domain GSM (TD-GSM). However, the realizable SE contributed by the constellation part remains unchanged from that of the traditional SM, which constrains the total achievable SE. In [15], the OFDM-based FD-generalized SMP (FD-GMP) and TD-generalized SMP (TD-GSMP) were proposed to improve the achievable SE. However, the system complexity is very high and it is also difficult to achieve a good trade-off between the SE and the BER performance, which is the critical challenge in practical applications. Furthermore, to enhance the reception performance, numerous research studies have been devoted to the design of favorable detection methods. A maximum-likelihood (ML) detection algorithm that jointly demodulates the transmitter serial number and the transmit symbols was proposed in [16], which required many iterations over all template information, therefore yielding an excellent BER performance. However, the ML detection algorithm possesses extremely high complexity. To solve this problem, the maximum ratio combining (MRC) method, which combines the signals by multiplying different coefficients to different signals of the diversity, is employed in MIMO systems. Compared to selective combining and equal gain combining, the MRC can both achieve better BER performance and have much lower complexity than the ML method [17; 18].